spi: s3c64xx: add missing check for polling mode

After the patch "spi/s3c64xx: Fix non-dmaengine usage"
with commit id 563b444e33810f3120838620c990480304e24e63
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>

drivers/spi/spi-s3c64xx.c

@@ -434,6 +434,9 @@ static int s3c64xx_spi_prepare_transfer(struct spi_master *spi)
 	dma_cap_mask_t mask;
 	int ret;
 
+	if (is_polling(sdd))
+		return 0;
+
 	dma_cap_zero(mask);
 	dma_cap_set(DMA_SLAVE, mask);