bnx2: Call pci_enable_msix() with actual number of vectors.

Based on original patch by Breno Leitão <leitao@linux.vnet.ibm.com>.

Allocate the actual number of vectors and make use of fewer vectors
if pci_enable_msix() returns > 0.  We must allocate one additional
vector for the cnic driver.

Cc: Breno Leitão <leitao@linux.vnet.ibm.com>
Reviewed-by: Benjamin Li <benli@broadcom.com>
Reviewed-by: Matt Carlson <mcarlson@broadcom.com>
Signed-off-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>