V4L/DVB (12407): em28xx: Adjust Silvercrest xtal frequency

We don't know the xtal frequency of Silvercrest, but we need to have
some value in order to allow controlling the frame rate frequency. The
value is probably still wrong, since the manufacturer announces this
device as being capable of 30fps, but the maximum we can get is
13.5 fps.

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