spi: sc18is602: Convert to let spi core validate transfer speed

Set master->max_speed_hz and master->min_speed_hz then spi core will handle
checking transfer speed. So we can remove the same checking in this driver.

This patch also remove testing if hz is 0 because spi->max_speed_hz will be
default set to master->min_speed_hz if it was not set. So the transfer speed
will never set to 0.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Signed-off-by: Mark Brown <broonie@linaro.org>