staging: iio: isl29028: only set ALS scale when ALS/IR sensing is enabled

isl29028_chip_init_and_power_on() calls isl29028_set_als_scale() and
this is not needed until the user actually needs to take a reading from
the ALS/IR sensor. This patch moves the isl29028_set_als_scale() call
from isl29028_chip_init_and_power_on() to isl29028_set_als_ir_mode().
This sets the stage for faster resume times from runtime power
management if the user is only querying the proximity sensor.

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