bnx2x: PFC changes

Change BRB to work in per class guaranteed mode and handle cases for BW 0%.

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>