alsa: mixer - add support up 8 mixer channels

alsa: mixer - add support up 8 mixer channels

We have at least one USB hardware which supports the 8
channels in one mixer element:

  https://github.com/alsa-project/alsa-ucm-conf/pull/25

POSITION_MASK_CHANNELS define was added for the future extensions.

The override_map variable was changed from bool to mask (unsigned int).
The channel map override settings is handled for channels up to eight now.

Also added missing override-map.3 .. override-map.8 to the configuration
parser array.

The driver channel position was added to the override mask arguments
(syntax is driver:pulseaudio like left:all-left). If ommited, the ALSA's
channel positions are guessed by index.

Link: https://gitlab.freedesktop.org/pulseaudio/pulseaudio/-/merge_requests/292
Signed-off-by: Jaroslav Kysela <perex@perex.cz>
Part-of: <https://gitlab.freedesktop.org/pulseaudio/pulseaudio/-/merge_requests/389>