media: gspca: fix g/s_parm handling

Fix v4l2-compliance error: s_parm never set V4L2_CAP_TIMEPERFRAME.
Also various g/s_parm-related cleanups.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>