spi: spi-fsl-dspi: Take software timestamp in dspi_fifo_write

Although the SPI system timestamps are supposed to reflect the moment
that the peripheral has received a word rather than the moment when the
CPU has enqueued that word to the FIFO, in practice it is easier to just
record the latter time than the former (with a smaller error).

With the recent migration of TCFQ users from poll back to interrupt mode
(this time for XSPI FIFO), it's wiser to keep the interrupt latency
outside of the measurement of the PTP system timestamp itself. If there
proves to be any constant offset that requires static compensation, that
can always be added later. So far that does not appear to be the case at
least on the LS1021A-TSN board, where testing shows that the phc2sys
offset is able to remain within +/- 200 ns even after 68 hours of
testing.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Link: https://lore.kernel.org/r/20200304220044.11193-13-olteanv@gmail.com


Signed-off-by: Mark Brown <broonie@kernel.org>