usb: gadget: remove net2280_set_fifo_mode()

This function has no user in my tree. It looks like it belongs to
net2280 but it somehow morphed into the dummy_hcd. So I remove it
before it spreads into more drivers.
After some digging I figured out that the only user was removed in

|commit 9079e91b
|Author: David Brownell <david-b@pacbell.net>
|Date:   Wed May 7 16:00:36 2008 -0700
|
|    USB: serial gadget: cleanup/reorg

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>