audiomixer: Should not take channel mask in consideration when in mono or stereo

Submitted by Mathieu Duponchelle @meh

Assigned to Mathieu Duponchelle @meh

Link to original bug (#708633)

Description

Created attachment 255577
fixes the reported issue

It can cause negotiation to fail.

Patch 255577, "fixes the reported issue":
0001-adder-Don-t-take-channel-mask-in-consideration-in-mo.patch