bnx2x: Prevent IOV if no entries in CAM

It's possible there's a bad chip configuration which will result with
PCIe IOV capabilities, but with no available interrupts for VFs.

In such case, we want to gracefully prevent the PF from initializing its
IOV capabilities rather than encounter difficulties further along the way.

Signed-off-by: Yuval Mintz <Yuval.Mintz@qlogic.com>
Signed-off-by: Ariel Elior <Ariel.Elior@qlogic.com>
Signed-off-by: David S. Miller <davem@davemloft.net>