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>

drivers/spi/omap2_mcspi.c

@@ -626,6 +626,12 @@ omap2_mcspi_txrx_pio(struct spi_device *spi, struct spi_transfer *xfer)
 		} else if (mcspi_wait_for_reg_bit(chstat_reg,
 				OMAP2_MCSPI_CHSTAT_EOT) < 0)
 			dev_err(&spi->dev, "EOT timed out\n");
+
+		/* disable chan to purge rx datas received in TX_ONLY transfer,
+		 * otherwise these rx datas will affect the direct following
+		 * RX_ONLY transfer.
+		 */
+		omap2_mcspi_set_enable(spi, 0);
 	}
 out:
 	omap2_mcspi_set_enable(spi, 1);