spi/s3c64xx: Fix compilation warning

Fix compilation warning by typecasting the tx_buf pointer.

[I'm not thrilled with resorting to a cast; but I cannot see a better
way to go about this.  I don't want to drop the const from struct
spi_transfer ~~glikely]

Signed-off-by: Jassi Brar <jassi.brar@samsung.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>