usb: dwc2: gadget: Fix Remote Wakeup interrupt bit clearing

To clear GINTSTS2_WKUP_ALERT_INT bit in GINTSTS2 register
require to write 1. This bit is implemented as "Write to clear".

Fixes: 187c5298 ("usb: dwc2: gadget: Add handler for WkupAlert interrupt")

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