ASoC: qdsp6v2: prevent null pointer dereference for _vol_cmds

In case of memory allocation failure, _vol_cmd_cnt is not reset.
In _volume_cmds_free, null pointer dereference would happen for
_vol_cmds[i].
To prevent it, reset _vol_cmd_cnt when memory allocation fails.

CRs-Fixed: 1089598
Change-Id: Icb998549cdb999c6db2fd52aef505f200e630da5
Signed-off-by: Xiaojun Sang <xsang@codeaurora.org>