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>