pcm: route: Don't handle no matching chmap as a serious error

pcm: route: Don't handle no matching chmap as a serious error

When find_matching_chmap() returns an error for the non-matching
chmap, the caller, snd_pcm_route_open(), also returns an error
although it shouldn't be handled as the fatal error.  This results in
the probe error with PulseAudio and it gives no real output in the
end.

Signed-off-by: Takashi Iwai <tiwai@suse.de>