usb: gadget: bcm63xx_udc: fix up the error of undeclared usb_debug_root

Fix up the build error caused by undeclared usb_debug_root

Cc: stable <stable@vger.kernel.org>
Fixes: a66ada4f

 ("usb: gadget: bcm63xx_udc: create debugfs directory under usb root")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Chunfeng Yun <chunfeng.yun@mediatek.com>
Signed-off-by: Felipe Balbi <balbi@kernel.org>