rpmsg: glink: Get reference of channel objects in rx path

Get channel references in data receive path as channel might get freed
while processing commands received from remote processor.

This ensures channel context is not freed before its usage is complete.

Change-Id: I7d9a98e34c21ae0d277456853a755dab8d105d5f
Signed-off-by: Pranav Mahesh Phansalkar <quic_pphansal@quicinc.com>
(cherry picked from commit a9ffd259)