usb: gadget: udc: net2280: remove full_speed module parameter

This patch removes the full_speed parameter used force full-speed
operation.

It was designed exclusively for testing purposes, and there is no
reason to maintain this in a production kernel.

Reverts: 2f076077
Suggested-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Ricardo Ribalda Delgado <ricardo.ribalda@gmail.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>