spi: atmel: Exposing effective spi speed

This patch implements the reporting of the effectively used speed_hz for
the transfer by setting xfer->effective_speed_hz.

See the following patch, which adds this feature to the SPI core for more
information:
commit 5d7e2b5e ("spi: core: allow reporting the effectivly used speed_hz for a transfer")

Signed-off-by: Thomas Kopp <thomas.kopp@microchip.com>
Reviewed-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Link: https://lore.kernel.org/r/20200921071036.2091-1-thomas.kopp@microchip.com


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