V4L/DVB (7733): blackbird_find_mailbox negative return ignored in blackbird_initialize_codec()

dev->mailbox is unsigned. so a negative return goes unnoticed

Signed-off-by: Roel Kluin <12o3l@tiscali.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>