igb: Correctly determine pci-e function number in virtual environment

When running in a virtual environment the ports of an 82575/6 can appear to
be single function devices which is not correct.  To resolve this we rely
on the function number stored in the status register.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>