usb: dwc2: gadget: Prevent handling of host interrupts

In host slave mode, the core asserts the rxready, txfifoempty interrupts
that get serviced in the gadget irq handler. Prevent servicing of these
when not in the gadget mode of operation.

Signed-off-by: Vardan Mikayelyan <mvardan@synopsys.com>
Signed-off-by: John Youn <johnyoun@synopsys.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>