igc: Remove unused _I_PHY_ID define

_I_PHY_ID not in use. Clean up the code accordingly,
and get rid of the unused define

Signed-off-by: Sasha Neftin <sasha.neftin@intel.com>
Tested-by: Nechama Kraus <nechamax.kraus@linux.intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>