usb: dwc2: gadget: free TX FIFO after killing requests

As kill_all_requests() potentially flushes TX FIFO, we should should
free FIFO after calling it. Otherwise FIFO could stay unflushed properly.

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>