staging: iio: isl29028: combine isl29028_proxim_get() and isl29028_read_proxim()

isl29028_proxim_get() checks to see if the promixity needs to be
enabled on the chip and then calls isl29028_read_proxim(). There
are no other callers of isl29028_read_proxim(). The naming between
these two functions can be confusing so this patch combines the
two to avoid the confusion.

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