usb: dwc2: host: ensure qtb exists before dereferencing it

dwc2_hc_nak_intr could be called with a NULL qtd.
Ensure qtd exists before dereferencing it to avoid kernel panic.
This happens when using usb to ethernet adapter.

Acked-by: John Youn <johnyoun@synopsys.com>
Signed-off-by: Gregory Herrero <gregory.herrero@intel.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>