msm: cvp: OOB write fix due to integer underflow

If FW send a pkt->size which is less than the sizeof packet structure
then pkt->size - sizeof() would result into an integer underflow.
Due to this the subsequent check would be bypassed and we will
start write to an OOB memory.

Change-Id: Icb3e4e6d64275592ceb6f747de653dcc1c65fec7
Signed-off-by: ptak <quic_ptak@quicinc.com>