Fix SPI struct unitialized use warning

@px4dev Another warning you might want to look at:

```
%% MODULE_MK           = /Users/user/src/Firmware/src/drivers/device/module.mk
%  MODULE_NAME         = device
%  MODULE_SRC          = /Users/user/src/Firmware/src/drivers/device/
%  MODULE_OBJ          = /Users/user/src/Firmware/Build/px4fmu-v1_default.build//Users/user/src/Firmware/src/drivers/device/module.pre.o
%  MODULE_WORK_DIR     = /Users/user/src/Firmware/Build/px4fmu-v1_default.build//Users/user/src/Firmware/src/drivers/device
CXX:     /Users/user/src/Firmware/src/drivers/device/cdev.cpp
CXX:     /Users/user/src/Firmware/src/drivers/device/device.cpp
CXX:     /Users/user/src/Firmware/src/drivers/device/i2c.cpp
CXX:     /Users/user/src/Firmware/src/drivers/device/pio.cpp
CXX:     /Users/user/src/Firmware/src/drivers/device/spi.cpp
In file included from /Users/user/src/Firmware/Build/px4fmu-v1_default.build/nuttx-export/include/arch/irq.h:60:0,
                 from /Users/user/src/Firmware/Build/px4fmu-v1_default.build/nuttx-export/include/nuttx/irq.h:70,
                 from /Users/user/src/Firmware/Build/px4fmu-v1_default.build/nuttx-export/include/nuttx/sched.h:54,
                 from /Users/user/src/Firmware/Build/px4fmu-v1_default.build/nuttx-export/include/sched.h:47,
                 from /Users/user/src/Firmware/Build/px4fmu-v1_default.build/nuttx-export/include/nuttx/arch.h:48,
                 from /Users/user/src/Firmware/src/drivers/device/spi.cpp:48:
/Users/user/src/Firmware/Build/px4fmu-v1_default.build/nuttx-export/include/arch/armv7-m/irq.h: In member function 'int device::SPI::transfer(uint8_t*, uint8_t*, unsigned int)':
/Users/user/src/Firmware/Build/px4fmu-v1_default.build/nuttx-export/include/arch/armv7-m/irq.h:293:29: warning: 'state' may be used uninitialized in this function [-Wmaybe-uninitialized]
/Users/user/src/Firmware/src/drivers/device/spi.cpp:136:13: note: 'state' was declared here
PRELINK: /Users/user/src/Firmware/Build/px4fmu-v1_default.build//Users/user/src/Firmware/src/drivers/device/module.pre.o
```