bnx2x: Pause settings

Pause settings
- 1G pause was not working due to missing write to the emac block
  (TX_MODE_FLOW_EN)
- The flow control should use the negotiated result (after autoneg) so
  we should save both the requested autoneg and the result
- The HW credits with flow control at 1G speed were not optimized and
  caused low throughput
- It is recommended to turn off flow control if the MTU is bigger than
  5000B due to internal buffers size

Signed-off-by: Yaniv Rosner <yanivr@broadcom.com>
Signed-off-by: Eilon Greenstein <eilong@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>