staging: iio: isl29028: remove nested if statements

There are two callers to the function isl29028_set_als_ir_mode() and
both instances use a nested if statement to only change the chip state
if it is not in the proper mode. This patch moves this check into the
isl29028_set_als_ir_mode() function to remove the nested if
statements.

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