net/mlx4_en: Disable global flow control when PFC enabled

Fix a bug when FC and PFC are enabled/disabled at the same time.
According to ConnectX-3 Programmer Manual these two features are mutial
exclusive.  So make sure when enabling PFC to turn off global FC and
vise versa.  Otherwise it hurts the performance.

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