[media] snd_tea575x: Add a cannot_mute flag

Some devices which use the tea575x tuner chip don't allow direct control
over the IO pins, and thus cannot mute the audio output.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
CC: Ondrej Zary <linux@rainbow-software.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>