mailbox: mpfs: fix copy and paste bug in probe

This code accidentally checks ->ctrl_base instead of ->mbox_base so the
error handling can never be triggered.

Fixes: a4123ffa ("mailbox: mpfs: support new, syscon based, devicetree configuration")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Jassi Brar <jassisinghbrar@gmail.com>