V4L/DVB (7163): em28xx: makes audio settings more stable

Improves audio configurations on em28xx:
        - mutes audio before changing amux;
        - adds a delay after setting audio src;
        - waits up to 50ms for ac97 busy.

Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>