ixgbe: use ETQF filter name instead of magic number

This patch removes a magic number that was used for the ETQF used for
filtering L2 ptp packets and replaces it with the supplied define that
previously existed. The intent is to clarify that this filter is already
set aside for L2 1588 work.

Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Tested-by: Phil Schmitt <phillip.j.schmitt@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>