V4L/DVB (12400): em28xx: Allow changing fps on webcams

em28xx doesn't have temporal scaling. However, on webcams, sensors are
capable of changing the output rate. So, VIDIOC_[G|S]_PARM ioctls should
be passed to the sensor for it to properly set frame rate.

Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>