net: stmmac: Correctly take timestamp for PTPv2

The case for PTPV2_EVENT requires event packets to be captured so add
this setting to the list of enabled captures.

Fixes: 891434b1

 ("stmmac: add IEEE PTPv1 and PTPv2 support.")
Signed-off-by: Jose Abreu <Jose.Abreu@synopsys.com>
Signed-off-by: David S. Miller <davem@davemloft.net>