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 5d7e2b5 ("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>
(cherry picked from commit 23f370c)
Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>

Author: Thomas Kopp

drivers/spi/spi-atmel.c

@@ -857,6 +857,7 @@ static int atmel_spi_set_xfer_speed(struct atmel_spi *as,
 	csr = spi_readl(as, CSR0 + 4 * chip_select);
 	csr = SPI_BFINS(SCBR, scbr, csr);
 	spi_writel(as, CSR0 + 4 * chip_select, csr);
+	xfer->effective_speed_hz = bus_hz / scbr;
 
 	return 0;
 }