[PATCH] Net2280 driver (2/6)

This patch creates drivers/usb/gadget/net2280.[hc],
providing a driver for NetChip's "Net2280 PCI USB 2.0
High Speed Peripheral Controller".

It implements the API included in the first patch.

The driver has behaved well with chiprev 0100 under
stress tests with Gadget Zero and the ethernet model
driver, and has passed sanity tests for chiprev 0110.