qrtr: Align header and payload

Remote IPC Router drivers expect the received packet to be aligned.
This case is only exposed using the MHI transport because it does not
ensure the packet will be received as one chunk like RPMSG GLINK.

Change-Id: I0e5c27e3c5896e6721c8bbbfcda29551f0968626
Signed-off-by: Chris Lew <clew@codeaurora.org>