hal: restore fm volume when codec backend config changed

During FM playback, usecase play-fm may be disabled and
re-routed because of codec backend config change. Default
FM volume is applied when usecase play-fm re-routed, thus
volume may be different from previous normal one. As for
current mixer path, default value for fm is too low to be
audible.

Restore FM volume after usecase play-fm re-routed by codec
backend config change.

CRs-Fixed: 2171323
Change-Id: I88117989ffcacc48268891c7144f78e7a9875c51