net: cdc_ncm: remove unused udev field

We already use the usbnet udev field everywhere this could have
been used.

Cc: Alexey Orishko <alexey.orishko@gmail.com>
Signed-off-by: Bjørn Mork <bjorn@mork.no>
Signed-off-by: David S. Miller <davem@davemloft.net>