docs: update documentation mistakes

Signed-off-by: Brendan Le Foll <brendan.le.foll@intel.com>
mraa_board_t: fix operator in *_mmap_unsetup

Signed-off-by: Brendan Le Foll <brendan.le.foll@intel.com>
gpio.c: fix incorrect debug message due to merge

Signed-off-by: Brendan Le Foll <brendan.le.foll@intel.com>
aio.c: remove less-than-zero comparison of an unsigned var

Signed-off-by: Brendan Le Foll <brendan.le.foll@intel.com>
aio.c: check aio_get_valid_fp return value

Signed-off-by: Brendan Le Foll <brendan.le.foll@intel.com>
gpio.c: check return status of open call

Signed-off-by: Brendan Le Foll <brendan.le.foll@intel.com>
gpio.c: fail early in case of error when reading file

Signed-off-by: Brendan Le Foll <brendan.le.foll@intel.com>
gpio.c: cast return value explicitly

Signed-off-by: Brendan Le Foll <brendan.le.foll@intel.com>
intel_de3815.c: make sure strncpy() doesn't go over 8 chars

Signed-off-by: Brendan Le Foll <brendan.le.foll@intel.com>
pwm.c: check error condition when doing read operations

Signed-off-by: Brendan Le Foll <brendan.le.foll@intel.com>
pwm.c: remove possibility of division by 0

Signed-off-by: Brendan Le Foll <brendan.le.foll@intel.com>