usb: gadget: dwc2: fix memory leak in gadget_init()

Freed allocated request for ep0 to prevent memory leak in case when
dwc2_driver_probe() failed.

Cc: Stefan Wahren <stefan.wahren@i2se.com>
Cc: Marek Szyprowski <m.szyprowski@samsung.com>
Tested-by: Stefan Wahren <stefan.wahren@i2se.com>
Tested-by: Marek Szyprowski <m.szyprowski@samsung.com>
Acked-by: Minas Harutyunyan <hminas@synopsys.com>
Signed-off-by: Grigor Tovmasyan <tovmasya@synopsys.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>