dwc_otg: Update NetBSD usb.h header licence

NetBSD have changed their licensing requirements such that the 2-clause
licence is preferred. Update usb.h in the downstream dwc_otg code
accordingly.

See https://www.netbsd.org/about/redistribution.html

 for more
information.

Signed-off-by: Phil Elwell <phil@raspberrypi.com>