usb: gadget: dummy_hcd: fix build when BUG is not set

Use WARN_ON() instead of __WARN, which also means we won't use any
internal macros.

Signed-off-by: Sasha Levin <levinsasha928@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>