usb: dwc3: gadget: re-factor Link state change to a function

Most link changes will, of course, happen with
the help of a matching host HW, but in some cases
we might want to debug very low level details about
the link and exposing this to debugfs sounds like
a good plan.

This is a preparation for such setup.

Signed-off-by: Felipe Balbi <balbi@ti.com>