net: ethernet: mlx4: Avoid assigning a value to ring_cons but not used it anymore in mlx4_en_xmit()

We found a set but not used variable 'ring_cons' in mlx4_en_xmit(), it will
cause a warning when build the kernel. And after checking the commit record
of this function, we found that it was introduced by a previous patch.

So, We delete this redundant assignment code.

Fixes: 488a9b48 ("net/mlx4_en: Wake TX queues only when there's enough room")

Signed-off-by: Luo Jiaxing <luojiaxing@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>