[ALSA] at73c213: monaural support

Add support for monaural playback to at73c213 driver.  The sound will be apear
on L-channel.  Tested on AT91SAM9260-EK.

Signed-off-by: Atsushi Nemoto <anemo@mba.ocn.ne.jp>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>