Fix IO Firmware build warnings

@px4dev These are the warnings I referred to:

```gcc
%% MODULE_MK           = /Users/user/src/Firmware/src/modules/px4iofirmware/module.mk
%  MODULE_NAME         = px4iofirmware
%  MODULE_SRC          = /Users/user/src/Firmware/src/modules/px4iofirmware/
%  MODULE_OBJ          = /Users/user/src/Firmware/Build/px4io-v1_default.build//Users/user/src/Firmware/src/modules/px4iofirmware/module.pre.o
%  MODULE_WORK_DIR     = /Users/user/src/Firmware/Build/px4io-v1_default.build//Users/user/src/Firmware/src/modules/px4iofirmware
CC:      /Users/user/src/Firmware/src/modules/px4iofirmware/adc.c
CC:      /Users/user/src/Firmware/src/modules/px4iofirmware/controls.c
CC:      /Users/user/src/Firmware/src/modules/px4iofirmware/dsm.c
CC:      /Users/user/src/Firmware/src/modules/px4iofirmware/px4io.c
CC:      /Users/user/src/Firmware/src/modules/px4iofirmware/registers.c
/Users/user/src/Firmware/src/modules/px4iofirmware/registers.c: In function 'registers_get':
/Users/user/src/Firmware/src/modules/px4iofirmware/registers.c:846:3: warning: assignment discards 'const' qualifier from pointer target type [enabled by default]
/Users/user/src/Firmware/src/modules/px4iofirmware/registers.c:863:3: warning: assignment discards 'volatile' qualifier from pointer target type [enabled by default]
/Users/user/src/Firmware/src/modules/px4iofirmware/registers.c:866:3: warning: assignment discards 'volatile' qualifier from pointer target type [enabled by default]
CC:      /Users/user/src/Firmware/src/modules/px4iofirmware/safety.c
CC:      /Users/user/src/Firmware/src/modules/px4iofirmware/sbus.c
/Users/user/src/Firmware/src/modules/px4iofirmware/sbus.c: In function 'sbus2_output':
/Users/user/src/Firmware/src/modules/px4iofirmware/sbus.c:131:1: warning: control reaches end of non-void function [-Wreturn-type]
/Users/user/src/Firmware/src/modules/px4iofirmware/sbus.c: In function 'sbus1_output':
/Users/user/src/Firmware/src/modules/px4iofirmware/sbus.c:124:1: warning: control reaches end of non-void function [-Wreturn-type]
CC:      /Users/user/src/Firmware/src/modules/px4iofirmware/../systemlib/up_cxxinitialize.c
CC:      /Users/user/src/Firmware/src/modules/px4iofirmware/../systemlib/perf_counter.c
CXX:     /Users/user/src/Firmware/src/modules/px4iofirmware/mixer.cpp
CXX:     /Users/user/src/Firmware/src/modules/px4iofirmware/../systemlib/mixer/mixer.cpp
CXX:     /Users/user/src/Firmware/src/modules/px4iofirmware/../systemlib/mixer/mixer_group.cpp
CXX:     /Users/user/src/Firmware/src/modules/px4iofirmware/../systemlib/mixer/mixer_multirotor.cpp
CXX:     /Users/user/src/Firmware/src/modules/px4iofirmware/../systemlib/mixer/mixer_simple.cpp
CC:      /Users/user/src/Firmware/src/modules/px4iofirmware/../systemlib/pwm_limit/pwm_limit.c
CC:      /Users/user/src/Firmware/src/modules/px4iofirmware/i2c.c
/Users/user/src/Firmware/src/modules/px4iofirmware/i2c.c:157:1: warning: no previous prototype for 'i2c_reset' [-Wmissing-prototypes]
/Users/user/src/Firmware/src/modules/px4iofirmware/i2c.c:72:14: warning: 'i2c_dump' declared 'static' but never defined [-Wunused-function]
PRELINK: /Users/user/src/Firmware/Build/px4io-v1_default.build//Users/user/src/Firmware/src/modules/px4iofirmware/module.pre.o
LINK:    /Users/user/src/Firmware/Build/px4io-v1_default.build/firmware.elf
BIN:     /Users/user/src/Firmware/Build/px4io-v1_default.build/firmware.bin
```