staging: iio: isl29018: use IIO_DEVICE_ATTR_{RO, RW} macros

Use the IIO_DEVICE_ATTR_RO and IIO_DEVICE_ATTR_RW macros to
create the device attributes.

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