iio: sx9500: fix bug in compensation code

The initial compensation was mistakingly toggling an extra bit in the
control register.  Fix this and make sure it's less likely to happen by
introducing an additional macro.

Reported-by: Hartmut Knaack <knaack.h@gmx.de>
Signed-off-by: Vlad Dogaru <vlad.dogaru@intel.com>
Acked-by: Hartmut Knaack <knaack.h@gmx.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>