ALSA: snd-atmel-ac97c: cleanup register definitions

This patch will remove traces of channel B registers, since they are not
used by the AC97C driver. Channel B might be used for other purposes.

The driver also adds channel status bits TXEMPTY and OVRUN and a
AC97C_CH_MASK macro to ease clearing a channel settings.

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