iio: bmp280: refactor compensation code

This version of the code avoids extra memory copy operations and is
somewhat smaller in code size.

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>