media: bcm2835-unicam: Fix a NULL vs IS_ERR() check

The media_pad_remote_pad_unique() function returns error pointers, not
NULL.  Update the check accordingly.

Fixes: 392cd78d

 ("media: bcm2835-unicam: Add support for CCP2/CSI2 camera interface")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Link: https://lore.kernel.org/r/fbbe7862-2820-44eb-81cb-7f33b99cca35@moroto.mountain
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>