spi: check tx_buf and rx_buf in spi_unmap_msg

spi: check tx_buf and rx_buf in spi_unmap_msg

Some spi device drivers use the same tx_buf and rx_buf repeatly for better
performance such as driver/input/touchsreen/ads7846.c, but spi core grab tx_buf
/rx_buf of transfer and set them as dummy_tx/dummy_rx once they are NULL. Thus,
in the second time the tx_buf/rx_buf will be replaced by dummy_tx/dummy_rx and
the data which produced by the last tx or rx may be wrongly sent to the device
or handled by the upper level protocol. This patch just keep the orignal value
of tx_buf/rx_buf if they are NULL after this transfer processed.

Signed-off-by: Robin Gong <b38343@freescale.com>
Signed-off-by: Mark Brown <broonie@kernel.org>