spi: s3c64xx: potential oops on probe error

We accidentally mixed up freeing the rx and tx channels which would a
leak and an oops.

Fixes: 3d63a47a ("spi: s3c64xx: Don't request/release DMA channels for each SPI transfer")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Marek Szyprowski <m.szyprowski@samsung.com>
Acked-by: Andi Shyti <andi.shyti@samsung.com>
Signed-off-by: Mark Brown <broonie@kernel.org>