net/mlx5e: Update xon formula

Set xon = xoff - netdev's max_mtu.
netdev's max_mtu will give enough time for the pause frame to
arrive at the sender.

Fixes: 0696d608

 ("net/mlx5e: Receive buffer configuration")
Signed-off-by: Huy Nguyen <huyn@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>