msm: adsprpc: Handle out of bounds access

Add check for channel ID in fastrpc_buf_alloc to
fix out of bounds access.

Change-Id: I92f2717edcc91c19309589ce7c11c91c8129b71a
Acked-by: Abhinav Parihar <parihar@qti.qualcomm.com>
Signed-off-by: Vamsi Krishna Gattupalli <quic_vgattupa@quicinc.com>