rpmsg: glink: Extract tx kick operation

Refactor out the tx kick operations to its own function, in preparation
for pushing the details to the individual transports.

Reviewed-by: Chris Lew <quic_clew@quicinc.com>
Signed-off-by: Bjorn Andersson <quic_bjorande@quicinc.com>
Signed-off-by: Bjorn Andersson <andersson@kernel.org>
Link: https://lore.kernel.org/r/20230213155215.1237059-2-quic_bjorande@quicinc.com