media: sta2x11: add a missing parameter description

Fixes this warning:
	drivers/media/pci/sta2x11/sta2x11_vip.c:156: warning: Function parameter or member 'v4l_lock' not described in 'sta2x11_vip'

Fixes: cd63c028 ("media: sta2x11: Add video_device and vb2_queue locks")
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>