ASoC: hdmi-codec: Fix broken channel map reporting

Commit 4e087133 ("ASoC: hdmi-codec: fix channel info for
compressed formats") accidentally changed hcp->chmap_idx from
ca_id, the CEA channel allocation ID, to idx, the index to
the table of channel mappings ordered by preference.

This resulted in wrong channel maps being reported to userspace,
eg for 5.1 "FL,FR,LFE,FC" was reported instead of the expected
"FL,FR,LFE,FC,RL,RR":

~ # speaker-test -c 6 -t sine
...
 0 - Front Left
 3 - Front Center
 1 - Front Right
 2 - LFE
 4 - Unknown
 5 - Unknown

~ # amixer cget iface=PCM,name='Playback Channel Map' | grep ': values'
  : values=3,4,8,7,0,0,0,0

Revert this incorrect change so that channel maps are properly
reported again.

Fixes: 4e087133

 ("ASoC: hdmi-codec: fix channel info for compressed formats")
Cc: stable@vger.kernel.org
Signed-off-by: Matthias Reichl <hias@horus.com>