PCI: check return value of pci_enable_device() when enabling bridges

pci_enable_device can fail. In that case, a printed warning would be
more appropriate.

Signed-off-by: Justin P. Mattock <justinmattock@gmail.com>
Signed-off-by: Junchang Wang <junchangwang@gmail.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>