iio:light:stk3310: make endianness independent of host

Data is stored in the device in be16 format. Make use of be16_to_cpu and
cpu_to_be16 to have correct endianness on any host architecture.

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