asoc: add missing null check for pcm pointer of snd_pcm_volume

In platform driver volume controls, add null check for
pcm pointer of struct snd_pcm_volume.

Change-Id: Icc139ae7dde0d45a3101b185ac69381df9e5e6c5
Signed-off-by: xsang <xsang@codeaurora.org>