amd-xgbe: Add a workaround for Tx timestamp issue

amd-xgbe: Add a workaround for Tx timestamp issue

Update the reading of the Tx timestamp to account for a hardware issue
on how the fields and interrupt are cleared.  The "seconds" portion of
the timestamp should be read first, followed by the "nanoseconds" portion.
Reading the "nanoseconds" portion should clear the timestamp data and the
interrupt.  Because of an issue with the hardware this order is reversed
and reading the "seconds" portion actually clears the timestamp.  The code
currently follows this workaround, but to guard against future versions
where this is fixed add a field to the version data to indicate if the
workaround is required or not.

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