usb: dwc3: gadget: don't use resource_index as a flag

We have a proper flag for testing that we have a valid transfer in
flight, let's use that instead.

This patch is in preparation to fix a rare race condition that happens
upon Disconnect Interrupt.

Tested-by: Thinh Nguyen <thinhn@synopsys.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>