sink, source: Assign to reference_volume from only one place

sink, source: Assign to reference_volume from only one place

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

Change-Id: I2e769b8a2b0d7031a02446dead8ca2e0c3402751
Signed-off-by: Jaska Uimonen <jaska.uimonen@intel.com>