usb: dwc3: drd: Don't free non-existing irq

If the driver is configured to use DRD role-switch, it's not OTG. There
won't be OTG irq to free. Check for dwc->otg_irq before freeing it.

Signed-off-by: Thinh Nguyen <thinhn@synopsys.com>
Signed-off-by: Felipe Balbi <balbi@kernel.org>