usb: dwc3: request 2mA VBUS current when suspending

In case a SUSPEND (L2 or U3) event is detected but the device is
not yet enumerated/configured, the device must still follow USB
power rules and limit its current draw to ISUSP (2.5mA). Notify
the gadget when this happens, which in turn will notify the
charger driver.

Change-Id: I10d890aabd78d841027b370eb62005d2e35e2a38
Signed-off-by: Jack Pham <jackp@codeaurora.org>