spi: dw: fix crash on setup stage

The commit 1a7b7ee7 (spi: Ensure that CS line is in non-active state after
spi_setup()) introduces an unconditional call of spi_set_cs() before ->setup().
The dw_spi_set_cs() relies on that fact that ->setup() is already called, but
it doesn't now. This patch fixes the crash by adding an additional check to
dw_spi_set_cs().
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>

drivers/spi/spi-dw.c

@@ -144,7 +144,7 @@ static void dw_spi_set_cs(struct spi_device *spi, bool enable)
 	struct chip_data *chip = spi_get_ctldata(spi);
 
 	/* Chip select logic is inverted from spi_set_cs() */
-	if (chip->cs_control)
+	if (chip && chip->cs_control)
 		chip->cs_control(!enable);
 
 	if (!enable)