[media] bttv: Improve TEA575x support

Improve g_tuner and add s_hw_freq_seek and enum_freq_bands support for cards
with TEA575x radio.

This allows signal/stereo detection and HW seek to work on these cards.

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@osg.samsung.com>