spi: rzv2m-csi: Adopt HZ_PER_MHZ for max spi clock

Make use of HZ_PER_MHZ for CSI_MAX_SPI_SCKO to make it clear
what its value means.

Signed-off-by: Fabrizio Castro <fabrizio.castro.jz@renesas.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Link: https://lore.kernel.org/r/20230715010407.1751715-3-fabrizio.castro.jz@renesas.com


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