net/mlx4_en: User prio mapping gets corrupted when changing number of channels

When using ethtool set_channels, mlx4_en_setup_tc is always called, even
when it was not configured. Fixed code to call mlx4_en_setup_tc() only
if needed.

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