[media] au8522: cleanup code which disables audio decoder

Cleanup the routine which disables the audio decoder.  In particular, the i2s
output was being shut off but the audio processor itself was not, resulting
in more power being consumed than needed.

Signed-off-by: Devin Heitmueller <dheitmueller@kernellabs.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>