usb: dwc2: gadget: reset fifo_map when initializing fifos

After all endpoints are disabled, fifo_map should have reached 0.
Its a bug if if didn't, so warn about it and reset it to 0 so that
driver can continue using all the fifos.

Tested-by: Robert Baldyga <r.baldyga@samsung.com>
Acked-by: Paul Zimmerman <paulz@synopsys.com>
Signed-off-by: Gregory Herrero <gregory.herrero@intel.com>
Signed-off-by: Mian Yousaf Kaukab <yousaf.kaukab@intel.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>