staging: iio: tsl2x7x_core: fix proximity treshold

Consider high byte of proximity min and max treshold in function
'tsl2x7x_chip_on'. So far, the high byte was not set.

Cc: Stable@vger.kernel.org
Signed-off-by: Mario Schuknecht <mario.schuknecht@dresearch-fe.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>