hal: Restore fm volume whenever enable fm usecase

During fm playback if there is a device switch
happening due to enable of another usecase, then fm volume
set by policy is lost when fm is resumed back. This is due to
disable and enable of the fm usecase and fm device resulting in
setting of default volume set from mixer_paths.

Change is to handle this scenario by resetting the actual fm
volume whenever fm usecase is enabled.

CRs-Fixed: 2256201
Change-Id: Ib11ab0fbbe6e038d41f53675adf3c4ca509692d0