mailbox: k3-sec-proxy: fix error handling for missing scfg in FDT

The wrong field was checked.

Fixes: f9aa4102 ("mailbox: Introduce K3 Secure Proxy Driver")
Signed-off-by: Matthias Schiffer <matthias.schiffer@ew.tq-group.com>
Reviewed-by: Nishanth Menon <nm@ti.com>