ALSA: oxygen: prevent muting of nonexistent AC97 controls

The Xonar DX does not have CD Capture controls, so we have to check that
a control actually exists before muting it.

Signed-off-by: Clemens Ladisch <clemens@ladisch.de>
Cc: <stable@kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>