4a-softmixer: bump to latest version

4a-softmixer: bump to latest version

Bug-AGL: SPEC-1533
This bump brings these commits:

* 731c21e (HEAD -> master, origin/master, origin/eel, origin/HEAD, eel) Rework the pcm copy loop and save a lot of CPU
* 7a02db1 plugins: code & log cleanup
* b124cc0 pcm core: fixed spurious XRUN issues
* aa13829 route plugin:fixed mismatch with several sound cards
* f8a4899 set the correct number of channels in dmix
* cf7b396 alsa-plug-vol: code & log cleanup
* 38f7eb2 alsa-utils-bypath: code & log cleanup
* 844108e .gitignore: ignore build with wildcard
* 30b321a alsa-utils-dump: added log upon failing parameters retrieval
* 407cfbe zones: fixed index mismatch when multiple sound cards are declared
* 3e00731 alsa-api-mixer: code & log cleanup
* 4c42a1d alsa-api-streams: code & log cleanup
* 12c0873 sinks: fixed index mismatch when multiple sound cards are declared
* 3b8dbd3 pcm: return volume value with null set request

Change-Id: I50c36477f538dabe62f2011241c351ba4a1d64f4
Signed-off-by: Thierry Bultel <thierry.bultel@iot.bzh>