media: v4l2: Use FIELD_SIZEOF directly

It's more clear  to use FIELD_SIZEOF instead of its implementation.

Signed-off-by: zhong jiang <zhongjiang@huawei.com>
Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>