spi: sirf: assign spi_master's max_speed_hz member

if spi device has no frequency, spi core will setup the default frequency
to max_speed_hz of spi_master according to
int spi_setup(struct spi_device *spi)
{
	...
        if (!spi->max_speed_hz)
                spi->max_speed_hz = spi->master->max_speed_hz;
	...
}
this patch moves CSR SiRFSoC SPI frequency set to follow SPI core behaviour.

Signed-off-by: Qipan Li <Qipan.Li@csr.com>
Signed-off-by: Barry Song <Baohua.Song@csr.com>
Signed-off-by: Mark Brown <broonie@kernel.org>