pci: Clean up error checking in pci_add_capability()

On success, pci_add_capability2() returns a positive value. On
failure, it sets an error and return a negative value.

pci_add_capability() laboriously checks this behavior. No other
caller does. Drop the checks from pci_add_capability().

Cc: mst@redhat.com
Cc: marcel@redhat.com
Signed-off-by: Mao Zhongyi <maozy.fnst@cn.fujitsu.com>
Reviewed-by: Marcel Apfelbaum <marcel@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
