igb: remove redundant PHY power down register write

One of the registers used to power down the PHY was found to be wrong
(should be bit 2 not bit 1) on further inspection it was also found to
be redundant.

Signed-off-by: Todd Fujinaka <todd.fujinaka@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>