V4L/DVB (9978): stkwebcam: Implement VIDIOC_ENUM_FRAMESIZES ioctl

This patch implements VIDIOC_ENUM_FRAMESIZES, it is used at least
by gstreamer.

Signed-off-by: Jaime Velasco Juan <jsagarribay@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>