usb: dwc3: Ignore multiple U3/L2 suspend events

The controller may sometimes generate back-to-back suspend
interrupt events. Ignore the repeated events to avoid calling
the gadget driver's suspend callback multiple times.

Change-Id: I284b461adeab5911388efd8bb7ecbf39a85c86ca
Signed-off-by: Jack Pham <jackp@codeaurora.org>