spi: ep93xx: 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. So we can remove the same checking in this driver.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Acked-by: Mika Westerberg <mika.westerberg@iki.fi>
Signed-off-by: Mark Brown <broonie@linaro.org>