tg3: Improve PTP TX timestamping logic

When we are trying to timestamp a TX packet, there may be
occasions when the TX timestamp register is still not
updated with the latest timestamp even if the timestamp
packet descriptor is marked as complete.
This usually happens in cases where the system is under
stress or flow control is affecting the transmit side.

We will solve this problem by saving the snapshot of the
timestamp register when we are posting the TX descriptor.
At this time, the register contains previously timestamped
packet's value and valid timestamp of the current packet must
be different than this.
Upon completion of the current descriptor, we will check if
the timestamp register is updated or not before timestamping
the skb. If not updated, we will schedule the ptp worker to
fetch the updated time later and timestamp the skb.
Also now we restrict number of outstanding PTP TX packet
requests to 1.

Reported-by: Simon White <Simon.White@viavisolutions.com>
Link: https://lore.kernel.org/netdev/CACKFLikGdN9XPtWk-fdrzxdcD=+bv-GHBvfVfSpJzHY7hrW39g@mail.gmail.com/


Signed-off-by: Pavan Chebbi <pavan.chebbi@broadcom.com>
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Reviewed-by: Andy Gospodarek <gospo@broadcom.com>
Reviewed-by: Kalesh AP <kalesh-anakkur.purayil@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>