usb: sam0: Implement missing API functions

This implements three API functions that are required for
tests/subsys/usb/device to build:

 - usb_dc_ep_disable()
 - usb_dc_ep_halt()
 - usb_dc_ep_flush()

While halt and disable are trivial, flush is just a stub for now.

Signed-off-by: Benjamin Valentin <benpicco@googlemail.com>