hdspmixer: add output loopback buttons

hdspmixer: add output loopback buttons

This patch adds "LPBK" buttons to the output strip
channels allowing to toggle the hardware output
loopback state.

Efforts were made to hide the buttons for not yet
enabled devices but due to difficulties in implementation
was cancelled for the time being. Should it be a no-go
efforts should be made to fix this and hide them,
although I would think fixing the driver for other
devices would be an effort with much more merit.

This is currently only enabled for HDSP9632 devices
with following driver patch:
https://github.com/tiwai/sound/commit/da2a040ee7cfe1dd57d5bec7906cb979c5787a86

We check whether the system has the kernel patch
applied by calling `HDSPMixerCard::supportsLoopback()`
and if not these buttons remain gray (off) and do
nothing when clicked.

Signed-off-by: Jasmin Fazlic <superfassl@gmail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>