spi: rspi: Fill in spi_transfer.effective_speed_hz

Fill in the effective bit rate used for transfers, so the SPI core can
calculate instead of estimate delays.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20200819125904.20938-7-geert+renesas@glider.be


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