spi: rspi: Fix NULL pointer dereference on SH7757

SH7757 doesn't pass platform data, in which case spi-rspi uses default
parameters.

However, commit 5ce0ba88 ("spi: rcar: add
Renesas QSPI support on RSPI") added a new user of the platform data, but
forgot to check for its validity first, causing a NULL pointer dereference
on SH7757.

Add the missing check to fix this.
Signed-off-by: Geert Uytterhoeven <geert+renesas@linux-m68k.org>
Acked-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Mark Brown <broonie@linaro.org>

drivers/spi/spi-rspi.c

@@ -958,8 +958,9 @@ static int rspi_probe(struct platform_device *pdev)
 	INIT_WORK(&rspi->ws, rspi_work);
 	init_waitqueue_head(&rspi->wait);
 
-	master->num_chipselect = rspi_pd->num_chipselect;
-	if (!master->num_chipselect)
+	if (rspi_pd && rspi_pd->num_chipselect)
+		master->num_chipselect = rspi_pd->num_chipselect;
+	else
 		master->num_chipselect = 2; /* default */
 
 	master->bus_num = pdev->id;