ALSA: 6fire: add a playback channel map

The driver registers no "Playback Channel Map" control, so a program
that needs the speaker position of each playback channel has to assume
an order; speaker-test, for example, assumes FL FR RL RR FC LFE for 6
channels. Channel n of the PCM goes to Line Out n, and the manual of
the card assigns Line Out 1 to 6 to front left and right, centre and
subwoofer, and rear left and right.

Add a playback channel map in the order of the manual,
FL FR FC LFE RL RR. A stream with 2 to 5 channels gets the first
positions of the list. A 1-channel stream gets MONO, as in every
1-channel entry of a channel map in sound/; the card plays it on Line
Out 1 only. The map has an entry for every channel count from 1 to 6,
because reading the control fails with -EINVAL for a count without an
entry.

Tested on a DMX 6Fire USB with kernel 7.2.10: during playback with 1,
3, 5 and 6 channels the control reads MONO, FL FR FC, FL FR FC LFE RL
and FL FR FC LFE RL RR, and the "Front Center" test of
speaker-test -c 6 played on Line Out 3.

Assisted-by: LLM
Signed-off-by: Piotr Gliźniewicz <p.glizniewicz@gmail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Link: https://patch.msgid.link/20261016073132.109288-3-p.glizniewicz@gmail.com

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