iio:accel:stk8312: rework macro definitions

Make use of BIT to describe register bits, GENMASK for consecutive
bitmasks, rename and sort existing definitions, replace magic value with
an expressive definition, drop an unused definition.

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>