USB: core: fix sparse warning for static function

Fix the following sparse warning:

drivers/usb/core/usb.c:1033:15: warning: symbol 'usb_debug_devices' was not declared. Should it be static?

Signed-off-by: Felipe Balbi <felipe.balbi@nokia.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>