igc: Fix wrong register name

Accordance to the i225 datasheet this register address
used by Host Transmit Discarded Packet by MAC counter
and not by not applicable Carrier Extension Error counter.
This patch comes to fix this wrong definition.

Signed-off-by: Sasha Neftin <sasha.neftin@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>