net: cdc_ncm: drop redundant driver-data assignment

The driver data for the data interface has already been set by
usb_driver_claim_interface() so drop the subsequent redundant
assignment.

Note that this also avoids setting the driver data three times in case
of a combined interface.

Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>