USB: mark USB drivers as being GPL only

Over two years ago, the Linux USB developers stated that they believed
there was no way to create a USB kernel driver that was not under the
GPL.  This patch moves the USB apis to enforce that decision.

There are no known closed source USB drivers in the wild, so this patch
should cause no problems.

Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>