staging: iio: isl29018: check if the chip is in a suspended state

Add a check to isl29018_write_raw() to ensure that the chip is not in a
suspended state. This makes the code consistent with what is present
in isl29018_read_raw().

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