staging:iio: Don't compare boolean values with true/false

Fixes the following coccicheck warnings:
	drivers/staging/iio/accel/lis3l02dq_ring.c:240:5-10: WARNING: Comparison to bool
	drivers/staging/iio/iio_dummy_evgen.c:111:6-25: WARNING: Comparison to bool

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