drivers: mellanox: use netdev_xmit_more() helper

skb->xmit_more hint is now always 0. This switches the mellanox drivers
to the netdev_xmit_more() helper.

Cc: Saeed Mahameed <saeedm@mellanox.com>
Cc: Leon Romanovsky <leon@kernel.org>
Cc: Boris Pismenny <borisp@mellanox.com>
Cc: Ilya Lesokhin <ilyal@mellanox.com>
Cc: Eran Ben Elisha <eranbe@mellanox.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: David S. Miller <davem@davemloft.net>