bnx2x: limit number of interrupt vectors for 57711

Original straightforward division may lead to zeroing number of SB and
null-pointer dereference when device is short of MSIX vectors or lacks
MSIX capabilities.

Reported-by: Vladislav Zolotarov <vladz@cloudius-systems.com>
Signed-off-by: Dmitry Kravkov <dmitry@broadcom.com>
Signed-off-by: Yuval Mintz <yuvalmin@broadcom.com>
Signed-off-by: Ariel Elior <ariele@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>