media: allegro: fix an error pointer vs NULL check

The allegro_mbox_init() function returns error pointers, it never
returns NULL.

Fixes: 94dc7656 ("media: allegro: rework mbox handling")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Michael Tretter <m.tretter@pengutronix.de>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>