igb: re-assign hw address pointer on reset after PCI error

Whenever the igb driver detects the result of a read operation returns
a value composed only by F's (like 0xFFFFFFFF), it will detach the
net_device, clear the hw_addr pointer and warn to the user that adapter's
link is lost - those steps happen on igb_rd32().

In case a PCI error happens on Power architecture, there's a recovery
mechanism called EEH, that will reset the PCI slot and call driver's
handlers to reset the adapter and network functionality as well.

We observed that once hw_addr is NULL after the error is detected on
igb_rd32(), it's never assigned back, so in the process of resetting
the network functionality we got a NULL pointer dereference in both
igb_configure_tx_ring() and igb_configure_rx_ring(). In order to avoid
such bug, this patch re-assigns the hw_addr value in the slot_reset
handler.

Reported-by: Anthony H Thai <ahthai@us.ibm.com>
Reported-by: Harsha Thyagaraja <hathyaga@in.ibm.com>
Signed-off-by: Guilherme G Piccoli <gpiccoli@linux.vnet.ibm.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>