All captured timestamps should be corrected by PHY, MAC and CDC introduced
latency/errors. The CDC correction is already used. Enable MAC propagation delay
correction as well which is available since commit 26cfb838 ("net: stmmac:
correct MAC propagation delay").

Before:
|ptp4l[390.458]: rms    7 max   21 freq   +177 +/-  14 delay   357 +/-   1

After:
|ptp4l[620.012]: rms    7 max   20 freq   +195 +/-  14 delay   345 +/-   1

Tested on Intel Elkhart Lake.
Signed-off-by: Kurt Kanzenbach <kurt@linutronix.de>
Reviewed-by: Johannes Zink <j.zink@pengutronix.de>
Signed-off-by: David S. Miller <davem@davemloft.net>