ixgbe: Don't report unsupported timestamping filters for X550

The current code enables on X550 timestamping of all packets for any
filter, which means ethtool should not report any PTP-specific filters
as unsupported.

Signed-off-by: Miroslav Lichvar <mlichvar@redhat.com>
Acked-by: Richard Cochran <richardcochran@gmail.com>
Acked-by: Jacob Keller <jacob.e.keller@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>