net/mlx5e: Fix xmit_more counter race issue

Update the xmit_more counter before notifying the HW,
to prevent a possible use-after-free of the skb.

Fixes: c8cf78fe

 ("net/mlx5e: Add ethtool counter for TX xmit_more")
Signed-off-by: Tariq Toukan <tariqt@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>