[media] em28xx: fix typo in scale_to_size()

em28xx: fix typo in scale_to_size().
The second hscale should be vscale. This bug caused xawtv to fail because it
cannot find a workable image size.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Acked-by: Frank Schäfer <fschaefer.oss@googlemail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>