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 <[e-mail]>
Signed-off-by: Mark Brown <[e-mail]>
Cc: <[e-mail]>

drivers/spi/spi-dw.c

···
 					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);
 }