stmmac: fix ptp header for GMAC3 hw timestamp

According the CYCLON V documention only the bit 16 of snaptypesel should
set.
(more information see Table 17-20 (cv_5v4.pdf) :
 Timestamp Snapshot Dependency on Register Bits)

Fixes: d2042052 ("stmmac: update the PTP header file")
Signed-off-by: Mario Molitor <mario_molitor@web.de>
Signed-off-by: David S. Miller <davem@davemloft.net>