firmware: arm_scmi: Fix NULL dereference on notify error path

firmware: arm_scmi: Fix NULL dereference on notify error path

Since commit b5daf93b809d1 ("firmware: arm_scmi: Avoid notifier
registration for unsupported events") the call chains leading to the helper
__scmi_event_handler_get_ops expect an ERR_PTR to be returned on failure to
get an handler for the requested event key, while the current helper can
still return a NULL when no handler could be found or created.

Fix by forcing an ERR_PTR return value when the handler reference is NULL.

Fixes: b5daf93b809d1 ("firmware: arm_scmi: Avoid notifier registration for unsupported events")
Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>
Reviewed-by: Dan Carpenter <dan.carpenter@linaro.org>
Message-Id: <20260305131011.541444-1-cristian.marussi@arm.com>
Signed-off-by: Sudeep Holla <sudeep.holla@kernel.org>