This is the first several autoconfig patches; please merge.
This particular one abstracts dual-speed (high and full)
support.


Support some more autoconfiguration for gadget drivers.

    Run-time:
	* Add gadget->is_dualspeed flag for controllers to set.
	* Tested by "ethernet" gadget, to decide whether certain
	  operations are errors or not.
	* Turned on by net2280.

    Compile-time
	* Generic CONFIG_USB_GADGET_DUALSPEED, not net2280-specific.
	* Used by "ethernet" gadget, to decide whether to
	  include extra code and data for dual-speed support.
	* Turned on by net2280.

The basic idea behind this, and other autoconfig patches yet to come,
is minimizing the controller-specific compile-time configuration
needed by gadget drivers.