usb: udc: Fix warnings on 64-bit builds

Cast u32 bit value to 64bit before recasting to 64bit pointer to avoid
pointer from integer cast size mismatch warnings.

Warning log:
+../drivers/usb/gadget/udc/udc-core.c: In function
‘usb_gadget_unmap_request’:
+../drivers/usb/gadget/udc/udc-core.c:68:19: warning: cast to pointer
from integer of different size [-Wint-to-pointer-cast]

Signed-off-by: Michal Simek <michal.simek@xilinx.com>