post_proc: return proper volume in reply data for offload effect.

Since EFFECT_FLAG_VOLUME_CTRL flag is set in non offload effect
implementations, EFFECT_CMD_SET_VOLUME should be handled in the same
manner as AOSP effect bundle.

In order for volume control to work, return the received volume in the
reply data.

CRs-Fixed: 2033812
Change-Id: Iee4cd3b80c1470034a208f0c5a014f10042a1ce2