staging: iio: isl29018: combine two return statements into one

Use the return value from isl29018_set_integration_time() as the return
value for isl29018_chip_init() since this is the last piece of work
inside that function.

Signed-off-by: Brian Masney <masneyb@onstation.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>