stmmac: fix PTP type ethtool stats

This patch fixes the ethtool stats for PTP frames; previous
version does not take care about some message types: i.e.
announce, management and signaling. It also provided a
broken statistic in case of "No PTP message received".

Signed-off-by: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Acked-by: Rayagond Kokatanur <rayagond@vayavyalabs.com>
Acked-by: Alexandre TORGUE <alexandre.torgue@st.com>
Acked-by: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>