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/20200917202420.1914104f

-1-mkl@pengutronix.de
Signed-off-by: Mark Brown <broonie@kernel.org>