PCI: fix uninitialized variable 'cap_mask'

Get rid of these:

drivers/pci/pcie/portdrv_core.c: In function 'pcie_port_device_register':
drivers/pci/pcie/portdrv_core.c:275:16: warning: 'cap_mask' may be used
uninitialized in this function [-Wuninitialized]
drivers/pci/pcie/portdrv_core.c:240:6: note: 'cap_mask' was declared here

In some cases, 'cap_mask' may be not set in pcie_port_platform_notify,
holding a garbage value.

Signed-off-by: Chunhe Lan <Chunhe.Lan@freescale.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>