pci: change offset type to uint16 in config space API.

* The config space is larger than 255, we need to use an uint16 to access
offsets superior or equal to 256. The current API only proposes an uint8 for this.
This change switches the offset parameter to the uint16 type. Axel hinted that
the used values are the same with such a change (the doc says sign extended to 2 or
4 bytes).
I checked with GCC2 and it's indeed the case when inspecting the memory.
With GCC4, instructions are the same on function call.
* prints info about extended capabilities.
* struct pci_module_info and struct pci_device_module_info are extended with
pci_find_extended_capability().