usb: gadget: r8a66597-udc: add support for set_selfpowered

The previous code always set to USB_DEVICE_SELF_POWERED in GET_STATUS.
So, this patch adds set_selfpowered().

Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>