firmware: arm_scmi: Fix raw coexistence mode behaviour on failure path

When SCMI raw coexistence mode is enabled make the core stack probe
successfully even when the initial base protocol exchanges with the
platform/server failed.

This behaviour enables the system to boot with a broken regular SCMI
stack but with a fully functional and accessible SCMI raw debugfs
interface that can be used to further debug the issue.

Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>
Link: https://lore.kernel.org/r/20230223152330.2707260-1-cristian.marussi@arm.com
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>