bnx2: Check BNX2_FLAG_USING_MSIX flag when setting up MSIX.

Checking the flag is more correct than checking bp->irq_nvecs. By
accident it is not a problem because we always have more than 1
vectors when using MSIX mode.

Signed-off-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: Benjamin Li <benli@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>