iio: Don't compare boolean values to true/false

Fixes the following warnings from coccicheck:
	drivers/iio/inkern.c:81:6-14: WARNING: Comparison to bool
	drivers/iio/dac/ad5686.c:191:5-11: WARNING: Comparison to bool

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>