[media] radio-aztech: Implement signal strength detection and fix stereo detection

Current stereo detection code is wrong - it reads TUNED bit instead of STEREO bit.
Fix that and implement signal strength detection too.
Also remove useless s_stereo functionn.

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