rpmsg: glink: Avoid infinite loop on intent for missing channel

In the event that an intent advertisement arrives on an unknown channel
the fifo is not advanced, resulting in the same message being handled
over and over.

Fixes: dacbb35e ("rpmsg: glink: Receive and store the remote intent buffers")
Signed-off-by: Bjorn Andersson <quic_bjorande@quicinc.com>
Reviewed-by: Chris Lew <quic_clew@quicinc.com>
Signed-off-by: Bjorn Andersson <andersson@kernel.org>
Link: https://lore.kernel.org/r/20230214234231.2069751-1-quic_bjorande@quicinc.com