igc: Remove UDP filter setup in PTP code

As implemented in igc_ethtool_get_ts_info(), igc only supports HWTSTAMP_
FILTER_ALL so any HWTSTAMP_FILTER_* option the user may set falls back to
HWTSTAMP_FILTER_ALL.

HWTSTAMP_FILTER_ALL is implemented via Rx Time Sync Control (TSYNCRXCTL)
configuration which timestamps all incoming packets. Configuring a
UDP filter, in addition to TSYNCRXCTL, doesn't add much so this patch
removes that code. It also takes this opportunity to remove some
non-applicable comments.

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>