ALSA: snd-atmel-ac97c: cleanup registers when removing driver

This patch will set the channel A and control channel mode register to
zero before disabling the AC97C peripheral.

Signed-off-by: Hans-Christian Egtvedt <hans-christian.egtvedt@atmel.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>