spi: spi-imx: spi_imx_transfer(): add support for effective_speed_hz

This patch implementes the reporting of the effectivly used speed_hz for the
transfer by setting tfr->effective_speed_hz.

See the following patch, which adds this feature to the SPI core for more
information:

    5d7e2b5e spi: core: allow reporting the effectivly used speed_hz for a transfer
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>
Link: https://lore.kernel.org/r/20200917202420.1914104-1-mkl@pengutronix.deSigned-off-by: Mark Brown <broonie@kernel.org>

drivers/spi/spi-imx.c

@@ -1503,6 +1503,8 @@ static int spi_imx_transfer(struct spi_device *spi,
 {
 	struct spi_imx_data *spi_imx = spi_master_get_devdata(spi->master);
 
+	transfer->effective_speed_hz = spi_imx->spi_bus_clk;
+
 	/* flush rxfifo before transfer */
 	while (spi_imx->devtype_data->rx_available(spi_imx))
 		readl(spi_imx->base + MXC_CSPIRXDATA);