spi: s3c64xx: add missing check for polling mode

Due to changes in mainline prior to submission the spi device detection
in polling mode breaks. This revealed the missing check for polling
during dma prepare. This patch adds the missing check.

Signed-off-by: Girish K S <ks.giri@samsung.com>
Signed-off-by: Mark Brown <broonie@linaro.org>