alsa-mixer: Allow selected fallback mappings in all profile sets
With !304 (merged) merged, when fallback mapping is selected all subsequent profile sets containing selected mapping are ignored. When there are only e.g. fallback input mappings available, admitted profile set will only contain one profile with selected first input fallback mapping and no outputs, and rest of profiles will only contain outputs and no inputs. When there are only fallback input and output mappings, there will be no profiles admitted at all.
Fix this by making sure that selected first fallback input or output mapping is actually allowed to exist in all probed profile sets.
Note while this change allows selected fallback mappings to be found in duplex configuraitons, probing fallbacks still can fail if there is more than one input fallback and first one (selected) does not work in duplex configurations.