igc: Fix MAX_ETYPE_FILTER value

The I225 controller has 8 ethertype filters, not 4. This patch fixes the
MAX_ETYPE_FILTER macro accordingly.

Signed-off-by: Andre Guedes <andre.guedes@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>