staging: iio: isl29028: only set proximity sampling rate when proximity is enabled

isl29028_chip_init_and_power_on() calls isl29028_set_proxim_sampling()
and this is not needed until the user actually needs to take a proximity
reading. This patch moves the isl29028_set_proxim_sampling() call from
isl29028_chip_init_and_power_on() to isl29028_enable_proximity().
This sets the stage for faster resume times from the runtime power
management if the user is only querying the ALS/IR sensor.

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