staging: iio: isl29018: fixed race condition in in_illuminance_scale_available_show()

in_illuminance_scale_available_show() references the
isl29018_chip->int_time variable in three places inside a for loop.
The value of the int_time variable can be updated by the
isl29018_set_integration_time() function, which is called by the
isl29018_write_raw() function. isl29018_write_raw() locks a
mutex specific to this driver when the integration time variable is
updated.

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