usb: dwc3: gadget: synchronize_irq dwc irq in suspend

We see dwc3 endpoint stopped by unwanted irq during
suspend resume test, which is caused dwc3 ep can't be started
with error "No Resource".

Here, add synchronize_irq before suspend to sync the
pending IRQ handlers complete.

Signed-off-by: Bo He <bo.he@intel.com>
Signed-off-by: Yu Wang <yu.y.wang@intel.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>