spi: s3c64xx: group the CS signalling writes in a single function

To enable/disable the CS line, the driver performs a writel in
the S3C64XX_SPI_SLAVE_SEL registers. Group the register's
configuration in a single function.

Signed-off-by: Andi Shyti <andi.shyti@samsung.com>
Signed-off-by: Mark Brown <broonie@kernel.org>