dwc2 always reports as self-powered in response to a device status
request.  Implement the set_selfpowered() operations so that the gadget
can report as bus-powered when appropriate.

This is modelled on the dwc3 implementation.
Acked-by: Minas Harutyunyan <hminas@synopsys.com>
Signed-off-by: John Keeping <john@metanate.com>
Signed-off-by: Felipe Balbi <balbi@kernel.org>