ALSA: AACI: fix number of channels for record

AC'97 codecs only support two channels for recording, so we shouldn't
advertize that there are up to six channels available.  Limit the
selection of 4 and 6 channel audio to playback only.

As this adds additional SNDRV_PCM_STREAM_PLAYBACK conditionals, we can
combine some resulting in the elimination of __aaci_pcm_open() entirely,
and making the code easier to read.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>