[media] em28xx: implement VIDIOC_ENUM_FRAMESIZES

Pidgin uses gstreamer (and libv4l) to work. Without implementing this ioctl,
it won't detect properly the size range, and driver will fail.

So, this patch is required, in order to use an em27xx webcam, like
Silvercrest.

The pigdin/gstreamer/libv4l needs to be fixed, as it shouldn't assume
that all drivers will implement this optional ioctl, but, at least now,
devices with em28xx have a better chance of working with pidgin.

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