usb: gadget: Delay low-power mode entry after USB bus suspend

When the USB bus is suspended, the suspend event handler immediately puts
the USB core into low power mode state. When this occurs concurrently with
other connectivity event, like disconnect, connect or reset, this may cause
race condition issues. This patch moves the low-power mode after suspend to
be handled by the OTG state machine which is capable of maintaining all of
the bus events in one location. Additionaly, a delay has been added between
the suspend event and the actual low-power mode entry.

CRs-fixed: 690236
Change-Id: I2239c27c0ed1d76b85e3ea7b02e6bbbd66a5b414
Signed-off-by: Danny Segal <dsegal@codeaurora.org>