usb: gadget: f_ncm: obtain cdev from function instead of driver_data

The 'driver_data' field in ep0 is never set to pointer to cdev, so we
have to obtain it from another source as in this context ep->driver_data
contains invalid data.

Signed-off-by: Robert Baldyga <r.baldyga@samsung.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>