mailbox: imx: Fix return in imx_mu_scu_xlate()

This called from mbox_request_channel().  The caller is  expecting error
pointers and not NULL so this "return NULL;" will lead to an Oops.

Fixes: 0a67003b ("mailbox: imx: add SCU MU support")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>