media: cobalt: drop static for sd_fmt

The struct v4l2_subdev_format sd_fmt cannot be static since it can be
written back by the subdev. Just have it on the stack.

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>