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

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:

    5d7e2b5ed585 spi: core: allow reporting the effectivly used speed_hz for a transfer

Signed-off-by: Marc Kleine-Budde <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Mark Brown <[email protected]>