firmware: arm_scmi: Account for failed debug initialization

When the SCMI debug subsystem fails to initialize, the related debug root
will be missing, and the underlying descriptor will be NULL.

Handle this fault condition in the SCMI debug helpers that maintain
metrics counters.

Fixes: 0b3d48c4 ("firmware: arm_scmi: Track basic SCMI communication debug metrics")
Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>
Message-Id: <20251014115346.2391418-1-cristian.marussi@arm.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>