amd-xgbe: Prevent looping forever if timestamp update fails

Just to be on the safe side, should the update of the timestamp registers
not complete, issue a warning rather than looping forever waiting for the
update to complete.

Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: David S. Miller <davem@davemloft.net>