ALSA: hda/realtek - Keep the channel count for multiple speakers

The current Realtek driver reconfigures the max PCM channels
dynamically according to the value of Channel Mode enum if the
multi-io retasking is available.  It works fine for multi-io pins.
But when multiple speaker pins are available, the channels of speakers
also have to obey to the channel mode, which isn't nice.
(That is, when you select "2ch" in Channel Mode so that the line-in
 and mic jack behave as input, you can't play surrounds properly from
 the built-in speaker.)

This patch fixes the problem by taking the channel number for multiple
speakers into account in the channel-mode setup code.
Also it fixes the wrongly set up max_channels value in the case of
multi-io extension.

Signed-off-by: Takashi Iwai <tiwai@suse.de>