spi: s3c64xx: Remove the 'set_level' callback from controller data

The set_level callback in the controller data, which is used to configure
the slave select line, cannot be supported when migrating the driver to
device tree based discovery. Since all the platforms currently use gpio
as the slave select line, this callback can be removed from the
controller data and replaced with call to gpio_set_value in the driver.

Signed-off-by: Thomas Abraham <thomas.abraham@linaro.org>
Acked-by: Jaswinder Singh <jaswinder.singh@linaro.org>
Acked-by: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>