usb: dwc3: gadget: use common is_selfpowered

Delete private selfpowered variable, and use common one.

Signed-off-by: Peter Chen <peter.chen@freescale.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>

diff --cc drivers/usb/dwc3/core.h
Simple merge

diff --cc drivers/usb/dwc3/ep0.c
Simple merge

diff --cc drivers/usb/dwc3/gadget.c
Simple merge