media: au0828: fix a NULL vs IS_ERR() check

The media_device_usb_allocate() function returns error pointers when
it's enabled and something goes wrong.  It can return NULL as well, but
only if CONFIG_MEDIA_CONTROLLER is disabled so that doesn't apply here.

Fixes: 812658d8 ("media: change au0828 to use Media Device Allocator API")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>