spi: dw: fix kernel crash due to NULL pointer dereference

The obvious fix after the commit d9c73bb8a3a5 "spi: dw: add support for gpio
controlled chip select". This patch fixes the issue by using locally defined
temporary variable.

Fixes: d9c73bb8a3a5 (spi: dw: add support for gpio controlled chip select)
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Mark Brown <broonie@linaro.org>
Cc: <stable@vger.kernel.org>

drivers/spi/spi-dw.c

@@ -271,7 +271,7 @@ static void giveback(struct dw_spi *dws)
 					transfer_list);
 
 	if (!last_transfer->cs_change)
-		spi_chip_sel(dws, dws->cur_msg->spi, 0);
+		spi_chip_sel(dws, msg->spi, 0);
 
 	spi_finalize_current_message(dws->master);
 }