usb: gadget: fix possible regression introduced with ep->claimed

This patch fixes possible regression introduced by
patch reworking endpoint claiming mechanism. It restores
setring ep->driver_data to NULL in usb_ep_autoconfig_reset(),
which was removed by patch commit cc476b42.

Fixes: cc476b42 ("usb: gadget: encapsulate endpoint
	claiming mechanism")
Reported-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Robert Baldyga <r.baldyga@samsung.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>