spi/omap2_mcspi: disable channel after TX_ONLY transfer in PIO mode

In the TX_ONLY transfer, the SPI controller also receives data
simultaneously and saves them in the rx register. After the TX_ONLY
transfer, the rx register will hold the random data received during
the last tx transaction.

If the direct following transfer is RX_ONLY, this random data has the
possibility to affect this transfer like this:
  When the SPI controller is changed from TX_ONLY to RX_ONLY,
  the random data makes the rx register full immediately and
  triggers a dummy write automatically(in SPI RX_ONLY transfers,
  we need a dummy write to trigger the first transaction).

  So the first data received in the RX_ONLY transfer will be that
  random data instead of something meaningful.

We can avoid this by inserting a Disable/Re-enable toggle of the
channel after the TX_ONLY transfer, since it purges the rx register.

Signed-off-by: Jason Wang <jason77.wang@gmail.com>
Tested-by: Grazvydas Ignotas <notasas@gmail.com>
Acked-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>