sink, source: Assign to s->muted from only one place

sink, source: Assign to s->muted from only one place

Forcing all mute changes to go through set_mute() makes it easier to
check where the muted field is changed, and it also allows us to have
only one place where notifications for changed mute are sent.

Change-Id: Idb1bd6ef923a165e249d42265ebedc30a6c8fca4
Signed-off-by: Jaska Uimonen <jaska.uimonen@intel.com>