usb: dwc3: fix sparse errors

sparse caught three mistakes on this driver,
fix them:

drivers/usb/dwc3/ep0.c:806:29: warning: duplicate const
drivers/usb/dwc3/debugfs.c:481:15: warning: symbol 'dwc3_debugfs_init' \
	was not declared. Should it be static?
drivers/usb/dwc3/debugfs.c:518:16: warning: symbol 'dwc3_debugfs_exit' \
	was not declared. Should it be static?

Signed-off-by: Felipe Balbi <balbi@ti.com>