netdev: Add n_txq to 'struct netdev'.

netdev: Add n_txq to 'struct netdev'.

This commit adds new variable n_txq to 'struct netdev' for recording
the number of tx queues.  Correspondingly, the send_*() functions are
extended to accept queue id as input argument.

All 'netdev-*' implementation will ignore the queue id since having
multiple tx queues is not supported.  Upcomping patches will start
using it and create multiple tx queues for dpdk netdev.

Signed-off-by: Alex Wang <alexw@nicira.com>
Acked-by: Pravin B Shelar <pshelar@nicira.com>