igc: Remove ethertype filter in PTP code

The driver only supports hardware timestamping for all incoming
traffic (HWTSTAMP_FILTER_ALL) which is enabled via Rx Time Sync
Control (TSYNCRXCTL) register already. Therefore, the ethertype
filter set in in igc_ptp_set_timestamp_mode() is useless so this
patch removes it.

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>