usb: gadget: at91udc: don't overwrite driver data

The driver was converted to the new start/stop interface in f3d8bf34
("usb: gadget: at91_udc: convert to new style start/stop interface").
I overlooked that the driver is overwritting the private data which is
used by the composite framework. The udc driver doesn't read it, it is
only written here.

Tested-by: Fabio Porcedda <fabio.porcedda@gmail.com>
Tested-by: Mario Isidoro <Mario.Isidoro@tecmic.pt>
Cc: <stable@kernel.org> # v3.5
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Felipe Balbi <balbi@ti.com>