spi: limit default transfer speed to controller's max speed

spi: limit default transfer speed to controller's max speed

Since the 'spi: Support transfer speed checking in the core'
change, the SPI core validates the desired speed of a given
transfer against the minimum and maximum speeds supported by
the controller.

If the speed of a transfer is not specified, the core uses
the maximum speed of the actual SPI device. However if the
maximum speed of the actual device is greater than the
maximum speed of the controller, the core will reject the
transfer due to the aforementioned change.

Change the code to use the maximum speed of the controller
by default if that is below the device's maximum speed.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Signed-off-by: Mark Brown <broonie@linaro.org>