ASoC: msm: qdsp6v2: Add support to set volume in ASM loopback

Allow to set volume in ASM for the loopback driver. Without the
get() function the corresponding volume mixer control is
failing to set volume.

CRs-Fixed: 1034862
Change-Id: I621dd9de3a8d4a0f4102227989e1dd17638c20ea
Signed-off-by: Surendar karka <sukark@codeaurora.org>