spi: omap2-mcspi: 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.

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