ALSA: hda - More Aspire 8930G fixes

Enable all three capture channels, including the missing nid 7 which is
the only one capable of capturing DMIC input

Enable Headphone amp for the HP jack. This causes a volume boost for
headphones, but does not cause any noticeable effect for light loads
like other amps, so there is no need to make it configurable.

Add Input Mix capture mux setting to capture the output of the playback
input mux (that is, what goes out the speakers except for PCM)

Hack another coef register because the stereo DMIC for some reason
produces a nonstandard sum/difference signal. I found a bit to make it
just use the sum signal for both channels, which makes it behave like a
standard mono microphone. The stereo is useless anyway (they're 1cm apart).

Tested working: Three capture channels, mic in, line in, DMIC.

Tested not working: CD. Not sure why, might be unconnected in the actual
hardware or a CD drive issue.

Also looked at SPDIF. It appears to work (emitter lights up inside the
HP out jack) but I lack a proper miniTOSLINK cable to test it.

Signed-off-by: Hector Martin <hector@marcansoft.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>