pch_gbe: correct receive time stamp filtering

This patch fixes the driver so that multicast PTP event messages can
be recognized by the hardware time stamping unit. The station address
register must be set according to the desired transport type.

[ RC - Rebased Takahiro's changes and wrote a commit message
  explaining the changes. ]

Signed-off-by: Takahiro Shimizu <tshimizu818@gmail.com>
Signed-off-by: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/ptp/Kconfig

@@ -78,9 +78,13 @@ config PTP_1588_CLOCK_PCH
 	depends on PCH_GBE
 	help
 	  This driver adds support for using the PCH EG20T as a PTP
-	  clock. This clock is only useful if your PTP programs are
-	  getting hardware time stamps on the PTP Ethernet packets
-	  using the SO_TIMESTAMPING API.
+	  clock. The hardware supports time stamping of PTP packets
+	  when using the end-to-end delay (E2E) mechansim. The peer
+	  delay mechansim (P2P) is not supported.
+
+	  This clock is only useful if your PTP programs are getting
+	  hardware time stamps on the PTP Ethernet packets using the
+	  SO_TIMESTAMPING API.
 
 	  To compile this driver as a module, choose M here: the module
 	  will be called ptp_pch.