spi: rspi: Remove useless .set_config_register() check

Not implementing spi_ops.set_config_register() is a driver bug that
would prevent the driver from working at all.
Hence remove the run-time check.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20200819125904.20938-2-geert+renesas@glider.be


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