net: cdc_ncm: remove redundant "disconnected" flag

Calling netif_carrier_{on,off} is sufficient.  There is no need
to duplicate the carrier state in a driver specific flag.

Acked-by: Enrico Mioso <mrkiko.rs@gmail.com>
Signed-off-by: Bjørn Mork <bjorn@mork.no>
Signed-off-by: David S. Miller <davem@davemloft.net>