rpmsg: glink: Fix GLINK command prefix

The upstream GLINK driver was first introduced to communicate with the
RPM on MSM8996, presumably as an artifact from that era the command
defines was prefixed RPM_CMD, while they actually are GLINK_CMDs.

Let's rename these, to keep things tidy. No functional change.

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/20230214225933.2025595-1-quic_bjorande@quicinc.com