usb: netusb: implement the ethernet interface.

net_eth_get_hw_capabilities() calls .get_capabilities() which is part
of the ethernet API but not the net_if_api that netusb implements.
This causes a fault as .get_capabilities() was past the end of the
struct and resolved to a random address.

Also, make the API const and give it a unique name.

Signed-off-by: Michael Hope <mlhx@google.com>