spi/spi_mpc8xxx: fix potential memory corruption.

spi/spi_mpc8xxx: fix potential memory corruption.

tx_dma/rx_dma are already set to a dummy buffer when no
tx/rx buffer and t->tx_dma/t->rx_dma does not contain a dma
address, but NULL.
This may lead to corruption of kernel memory. Fix this by
leaving tx_dma/rx_dma alone.

Do not INIT_TX_RX while controller is enabled, this is bad according
to the MPC8321 manual.

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>