V4L/DVB (4727): Support status readout for saa713x based FM radio

This patch adds readout for stereo and signal level for
saa713x cards which use the saa713x as FM demodulator.
These are many cards based on saa7133, tda8290 and tda8275a.
FM channel search should work now.

Signed-off-by: Hartmut Hackmann <hartmut.hackmann@t-online.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>