USB: GADGET: optionally force full-speed for net2280 UDC

This patch (as1656) adds a module parameter to the net2280 UDC driver
to force full-speed operation.  It is intended for testing purposes,
where one wants to check how well a full-speed device performs when
attached to a high-speed bus.  Without this parameter it would be
necessary to interpose a full-speed hub; otherwise the net2280 would
connect at high speed.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Acked-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>