V4L/DVB (12104): ivtv/cx18: fix regression: class controls are no longer seen

A previous change (v4l2-common: remove v4l2_ctrl_query_fill_std) broke
the handling of class controls in VIDIOC_QUERYCTRL. The MPEG class control
was broken for all drivers that use the cx2341x module and the USER class
control was broken for ivtv and cx18.

This change adds back proper class control support.

Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>