staging: iio: isl29018: change isl29018_read_raw() to only have one exit point

When the chip is in a suspended state, isl29018_read_raw() will return
-EBUSY. Change the function so that it only has a single exit point.

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