bnx2x: add missing parenthesis to prevent u32 overflow

Commit b475d78f

 lacked two pairs of parenthesis, causing an overflow in the
congestion management.

Signed-off-by: Dmitry Kravkov <dmitry@broadcom.com>
Signed-off-by: Yuval Mintz <yuvalmin@broadcom.com>
Signed-off-by: Eilon Greenstein <eilong@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>