spi: nuc900: Fix mode_bits setting

The code in nuc900_slave_select() supports handling SPI_CS_HIGH.
Thus set SPI_CS_HIGH bit in master->mode_bits to make it work.
Otherwise, spi_setup() will return unsupported mode bits error message if
SPI_CS_HIGH is set in the mode field of struct spi_device.

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