igc: Remove obsolete mask

'IGC_CTRL_EXT_LINK_MODE_MASK' not in use. This patch comes to tidy up
obsolete 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>