usb: dwc3: gadget: always use frame number from XferNotReady

The core requires the extra two bits of information for properly
scheduling Isochronous transfers. This means that we can't rely on
__dwc3_gadget_get_frame(). Let's always cache uFrame number from
XferNotReady instead.

Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>