bnx2x: fix handling single MSIX mode for 57710/57711

commit 30a5de77 added
ability to use single MSI-X vector, but lack proper
handling for 57710/57711 HW

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