firmware: arm_scmi: Fix NULL dereference on notify error path

In the Linux kernel, the following vulnerability has been resolved:

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.