spi: txx9: Convert to let spi core handle checking transfer speed

By setting master->max_speed_hz and master->min_speed_hz, spi core will handle
checking transfer speed. Then we can remove the same checking in this driver.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Reviewed-by: Atsushi Nemoto <anemo@mba.ocn.ne.jp>
Signed-off-by: Mark Brown <broonie@linaro.org>