igb: support BCM54616 PHY

The management port on an Edgecore AS7712-32 switch uses an igb MAC, but
it uses a BCM54616 PHY. Without a patch like this, loading the igb
module produces dmesg output like this:

[    3.439125] igb: Copyright (c) 2007-2014 Intel Corporation.
[    3.439866] igb: probe of 0000:00:14.0 failed with error -2

Signed-off-by: John W Linville <linville@tuxdriver.com>
Cc: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>