[AE][ALSA] Fix support for cards with hardware mixer

Cards with hardware mixer doesn't need to use dmix at all.
Old behavior leads to problem with playing non 48kHz audio on
multispeaker configuration.
On Audigy wrong device was opened according to frequency:
44.1kHz: front:CARD=Audigy,DEV=0
48.0kHz: sysdefault:CARD=Audigy

This patch enables check for hardware mixer availability and
open PCM device with taking it into account.

More information: http://trac.xbmc.org/ticket/13381