V4L/DVB (11275): tvaudio: fix mute and s/g_tuner handling

The mute control depends on CHIP_HAS_INPUTSEL, so test for that first.

The s/g_tuner code should check whether getmode/setmode is set at the
beginning instead of filling in the struct and discovering at
the end that this chip doesn't implement audiomodes after all (i.e. is
a simple muxer chip).

Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>