net: stmmac: fix PTP packet configuration

Snapshot bit being set before was selectively timestamping some packets.
Updated the snapshot bit to timestamp all type of PTP packets.
Adjust/init systime was not checking for previous incomplete updates.
Added check to fix it.

Change-Id: I2cfa63421fb8d1f36cab02e2e2fc5e3be1f28ac3
Signed-off-by: Sneh Shah <snehshah@codeaurora.org>