usb: dwc2: gadget: use ep->fifo_index in context of FIFO registers

In context of FIFO registers we use ep->fifo_index instead of ep->index.

Signed-off-by: Robert Baldyga <r.baldyga@samsung.com>
Signed-off-by: John Youn <johnyoun@synopsys.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>