usb: dwc2: gadget: remove dead code from dwc2_hsotg_ep_enable()

Since FIFO is always freed in dwc2_hsotg_ep_disable(), ep->fifo_index
is always 0 in dwc2_hsotg_ep_enable(), hence code inside if() block is
never executed.

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>