[media] saa7134: v4l2-compliance: return real frequency

Make saa7134 driver more V4L2 compliant: don't cache frequency in
s_frequency/g_frequency but return real one instead

Signed-off-by: Ondrej Zary <linux@rainbow-software.org>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>