spi: atcspi200: avoid NULL dereference

For SPI_XFER_BEGIN | SPI_XFER_END the code sets data_out = NULL.
In the debug statement we should not dereference this value.
As we do not transfer any data the debug statement is not needed in this
case anyway.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Jagan Teki <jagan@openedev.com>