igc: Remove unused phy_type enum

Complete to commit 8e153faf ("igc: Remove unused phy type")
i225 parts have only one PHY. There is no point to use phy_type enum.
Clean up the code accordingly, and get rid of the unused enum lines.

Signed-off-by: Sasha Neftin <sasha.neftin@intel.com>
Tested-by: Naama Meir <naamax.meir@linux.intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>