mailbox: bcm2835: Fix of_xlate return value

The bcm2835-mailbox returns NULL instead of an error pointer, which could
result in a NULL ptr dereference in mbox_request_channel. So fix this
by returning a proper error pointer.

Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Fixes: 0bae6af6 ("mailbox: Enable BCM2835 mailbox support")
Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>