spi/spi_mpc8xxx: Do not use map_tx_dma to unmap rx_dma

This fixes a typo were map_tx_dma is used instead of
map_rx_dma, casing the driver to unmap rx_dma when it
shouldn't.

Signed-off-by: Joakim Tjernlund <Joakim.Tjernlund@transmode.se>
Acked-by: Anton Vorontsov <cbouatmailru@gmail.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>

drivers/spi/spi_mpc8xxx.c

@@ -515,7 +515,7 @@ static void mpc8xxx_spi_cpm_bufs_complete(struct mpc8xxx_spi *mspi)
 
 	if (mspi->map_tx_dma)
 		dma_unmap_single(dev, mspi->tx_dma, t->len, DMA_TO_DEVICE);
-	if (mspi->map_tx_dma)
+	if (mspi->map_rx_dma)
 		dma_unmap_single(dev, mspi->rx_dma, t->len, DMA_FROM_DEVICE);
 	mspi->xfer_in_progress = NULL;
 }