PCI: Return u16 from pci_find_ext_capability() and similar

PCI Express Extended Capabilities are in config space between offsets 256
and 4K.  These offsets all fit in 16 bits.

Change the return type of pci_find_ext_capability() and supporting
functions from int to u16 to match the specification.  Many callers use
"int", which is fine, but there's no need to store more than a u16.

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