spi: s3c64xx: add missing check for polling mode

After the patch "spi/s3c64xx: Fix non-dmaengine usage"
with commit id 563b444e


submitted by Mark Brown, 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>