usb: gadget: pch_udc: Fix USB suspend issue

ISSUE:
After USB Suspend, a system rarely freezes.

CAUSE:
When USB Suspend occurred, the driver is not notifying
a gadget of the event.

Signed-off-by: Tomoya MORINAGA <tomoya.rohm@gmail.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>