media: camss: Replace .open() file operation with v4l2_fh_open()

media: camss: Replace .open() file operation with v4l2_fh_open()

The custom video_open() function in the camss driver open-codes the
v4l2_fh_open() helper, with an additional mutex that protects the whole
function. Given that the function does not modify any data guarded by
the lock, there's no need for using the mutex and the function can be
replaced by v4l2_fh_open().

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Hans Verkuil <hverkuil+cisco@kernel.org>