spi/s3c64xx: improve error handling

When a device tree definition os an s3c64xx SPI master is missing
a "controller-data" subnode, the newly added s3c64xx_get_slave_ctrldata
function might use uninitialized memory in place of that node,
which was correctly reported by gcc.

Without this patch, building s3c6400_defconfig results in:

drivers/spi/spi-s3c64xx.c: In function 's3c64xx_get_slave_ctrldata.isra.25':
drivers/spi/spi-s3c64xx.c:841:5: warning: 'data_np' may be used uninitialized in this function [-Wuninitialized]

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