include: trace: Fix inflight count helper on failed initialization

[ Upstream commit 289ce7e9 ]

Add a check to the scmi_inflight_count() helper to handle the case
when the SCMI debug subsystem fails to initialize.

Fixes: f8e65638 ("include: trace:  Add tracepoint support for inflight xfer count")
Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>
Message-Id: <20251014115346.2391418-2-cristian.marussi@arm.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>