media: uvcvideo: Drop stream->mutex

Since commit c93d73c9 ("media: uvcvideo: Use vb2 ioctl and fop
helpers"), the IOCTLs are serialized. Due to this there is no more need
to protect ctrl, cur_format or cur_frame from concurrent access.

Drop stream->mutex after thanking it for years of good service.

Use this opportunity to do fix some CodeStyle.

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