spi: spi-sh-msiof: set hi/low Active for HW CS

Set hardware CS(CS control function on MSIOF <-> GPIO CS) polarity
according to SPI_CS_HIGH flag on spi->mode.

Signed-off-by: Takashi Yoshii <takasi-y@ops.dti.ne.jp>
Signed-off-by: Mark Brown <broonie@linaro.org>