move pci_find_ext_capability() into common PCI code

move pci_find_ext_capability() into common PCI code

There's nothing architecture specific about it. It requires, however,
that x86-32's pci_conf_read32() tolerates register accesses above 255
(for consistency the adjustment is done to all pci_conf_readNN()
functions).

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Keir Fraser <keir@xen.org>