rpmsg: glink: Tidy up RX advance handling

The operation of advancing the FIFO receive pointer is sprinkled between
the interrupt handler itself, and functions being called from this.

Push all the RX advancement operations to the individual handlers, to
unify the style across the handling of the various messages.

Signed-off-by: Bjorn Andersson <quic_bjorande@quicinc.com>
Reviewed-by: Chris Lew <quic_clew@quicinc.com>
Link: https://lore.kernel.org/r/20240805-glink-tracepoints-v1-1-a5f3293fb09e@quicinc.com


Signed-off-by: Bjorn Andersson <andersson@kernel.org>