iio: light: STK3310: un-invert proximity values

In accordance with the recent proximity ABI changes,
STK3310's proximity readings should be un-inversed
in order to return low values for far-away objects
and high values for close ones.

As consequences of this change, iio event directions
have been switched and maximum proximity sensor
reference values have also been adjusted in accordance
with the real readings.

Signed-off-by: Tiberiu Breana <tiberiu.a.breana@intel.com>
Acked-by: Hartmut Knaack <knaack.h@gmx.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>