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

Set master->max_speed_hz then spi core will handle checking transfer speed.
The behavior is different from current code when the speed_hz is greater than
the maximum transfer speed supported by the controller.
Unless there is other good reason, I think we had better make the behavior
consistent with what spi core does.

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