igb: fix PHY config access on 82580

82580 NICs can have up to 4 functions. This fixes phy accesses
to use the correct locks for functions 2 and 3.

Signed-off-by: Nicholas Nunley <nicholasx.d.nunley@intel.com>
Tested-by: Jeff Pieper <jeffrey.e.pieper@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>