usb: dwc3: gadget: ignore suspend interrupt before connect

The controller sees a suspend event prior to the the cable connect
or reset events. As a result, it runs through an unnecessary
suspend sequence. Adding a check on the gadget's state avoids
the overhead.
The root cause of this appears to be a situation where the
controller senses/is told that there is a VBUS, but the port
reset is not forthcoming. In such a case, the line state is
interpreted as suspend, and the event is generated.
When VBUS detach is sensed/indicated, the state is returned to
USB_STATE_NOTATTACHED, in preparation for the next cable connect.

Change-Id: I639454ceb0ad3a8ff2c02d527c553a934b8639fc
Signed-off-by: Dov Levenglick <dovl@codeaurora.org>