spi: core: Use master->max_speed_hz as transfer speed when xfer->speed_hz > master->max_speed_hz

When xfer->speed_hz is greater than master->max_speed_hz, it's generally safe
to use master->max_speed_hz as transfer speed.
Thus use master->max_speed_hz as transfer speed rather than return error when
xfer->speed_hz > master->max_speed_hz.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Tested-by: Guenter Roeck <linux@roeck-us.net>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Mark Brown <broonie@linaro.org>