Fix crash in linux PF driver when BARs have been cleared/de-programmed;
fail early init (prior to mapping BARs) if the BAR0 or
BAR1 registers are zero.

This situation can arise when the PF is added to a VM (PCI pass-through),
then a PF FLR is issued (in the VM).  After this occurs, the BAR registers
will be zero. If we attempt to load the PF driver in the host
(after VM has been shutdown), the host can reset.
Signed-off-by: Rick Farrington <ricardo.farrington@cavium.com>
Signed-off-by: Raghu Vatsavayi <raghu.vatsavayi@cavium.com>
Signed-off-by: Felix Manlunas <felix.manlunas@cavium.com>
Signed-off-by: David S. Miller <davem@davemloft.net>