rpmsg: glink: Fail qcom_glink_tx() once remove has been initiated

Upon removing the glink edge, communication is at best one-way. This
means that the very common scenario of glink requesting intents will not
be possible to serve.

Typically a successful transmission results in the client waiting for a
response, with some timeout and a mechanism for aborting that timeout.

Because of this, once the glink edge is defunct once removal is
commenced it's better to fail transmissions fast.

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/20230213155215.1237059-6-quic_bjorande@quicinc.com