igc: Remove temporary workaround

PHY_CONTROL register works as defined in the IEEE 802.3 specification
(IEEE 802.3-2008 22.2.4.1). Tidy up the temporary workaround.

User impact: PHY can now be powered down when the ethernet link is down.

Testing hints: ip link set down <device> (or just disconnect the
ethernet cable).

Oldest tested NVM version is: 1045:740.

Fixes: 5586838f ("igc: Add code for PHY support")
Signed-off-by: Sasha Neftin <sasha.neftin@intel.com>
Reviewed-by: Paul Menzel <pmenzel@molgen.mpg.de>
Tested-by: Naama Meir <naamax.meir@linux.intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>