ASoC: msm-pcm-voip: Avoid interger underflow

There is no check for voip pkt pkt_len,if it contains the
minimum required data. This can lead to integer underflow.
Add check for the same.

Change-Id: I4f57eb125967d52ad8da60d21a440af1f81d2579
Signed-off-by: Soumya Managoli <quic_c_smanag@quicinc.com>