4a-hal-generic: aborting with "Can't enter dispatch while in dispatch!" when a phone is connected via bluetooth

When a phone is connected, 4a ends aborts, with the following error:

"Can't enter dispatch while in dispatch!"

After analysis, this is because a sync call (dynamic creation of the needed streams in softmixer) is performed. Replacing the afb_api_call_sync by a afb_api_call fixes it.