iio: accel: mma9553: refactor mma9553_read_raw

Refactor code for simplicity and clarity.

Signed-off-by: Irina Tirdea <irina.tirdea@intel.com>
Suggested-by: Hartmut Knaack <knaack.h@gmx.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>