ALSA: asihpi - Use valid channel count in format enumeration.

Since introduction of mono and low latency modes, fixed channel count of 2
is not always valid.  Use reported max_channels instead.

Signed-off-by: Eliot Blennerhassett <eblennerhassett@audioscience.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>