audioflinger: Add missing code for FM volume

 * If we used a proper MR2 audio HAL, this wouldn't be needed,
   but building FM support with the hacked up MR1 audio HAL we
   uses fails miserably. Adding this back fixes that.

Change-Id: If2f6ee9e417732eaf4df8718bf73b2dfd1062011
Merge pull request #5 from omnirom/android-4.4

audioflinger: Add missing code for FM volume