spi: orion: Let spi core handle checking transfer speed

Since commit a2fd4f9f 'spi: Support transfer speed checking in the core',
the SPI core validates the desired speed of a given transfer against the
minimum and maximum speeds supported by the controller.
So we can remove the same code in this driver and let spi core handle checking
the desired speed of a given transfer.

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