net/mlx4_en: Fix TX moderation info loss after set_ringparam is called

We need to re-set tx moderation information after calling set_ringparam
else default tx moderation will be used.
Also avoid related code duplication, by putting it in a utility function.

Signed-off-by: Amir Vadai <amirv@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>