media: i2c: mt9p031: Rewrite assignment to make smatch happy

media: i2c: mt9p031: Rewrite assignment to make smatch happy

The current code makes smatch a bit uncomfortable:
drivers/media/i2c/mt9p031.c:799 mt9p031_s_ctrl() warn: assigning (-1952) to unsigned variable 'data'

Probably because smatch is not clever enough (yet). Do a simple rewrite
to make sure that smatch understands what we are doing here.

Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Ricardo Ribalda <ribalda@chromium.org>
Signed-off-by: Hans Verkuil <hverkuil+cisco@kernel.org>