usb: dwc3: core: add fifo space helper

this helper will be used, initially, to dump space
of different queues and fifos in dwc3 to
debugfs. Later, it'll be used to issue remote wakeup
when we want to start a transfer and there's
something in a TX FIFO.

Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>