PCI: use pci_find_ext_capability everywhere

Remove some open coded (and buggy) versions of pci_find_ext_capability
in favor of the real routine in the PCI core.

Tested-by: Tomasz Czernecki <czernecki@gmail.com>
Acked-by: Andrew Vasquez <andrew.vasquez@qlogic.com>
Reviewed-by: Matthew Wilcox <willy@linux.intel.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>