usb: dwc2: gadget: change variable name to more meaningful

Since we handle FIFOs and endpoint separately, using variable named 'ep'
in context of FIFO is misleading, hence we rename it to 'fifo'.

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>