caif_usb: Check driver name before reading driver state in netdev notifier

In cfusbl_device_notify(), the usbnet and usbdev variables are
initialised before the driver name has been checked.  In case the
device's driver is not cdc_ncm, this may result in reading beyond the
end of the netdev private area.  Move the initialisation below the
driver name check.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>