[media] cobalt: fix unchecked return values

Function pci_find_ext_capability() may return 0, which is an invalid
address. In function cobalt_pcie_status_show(), its return value is used
without validation. This patch adds checks to validate the return
address.

Signed-off-by: Pan Bian <bianpan2016@163.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>