spi: dw: Avoid BUG_ON() in case of host failure

If dws is NULL in dw_spi_host_add(), we return the error to the
upper callers instead of crashing. The patch replaces BUG_ON by
returning -EINVAL to the caller.
Signed-off-by: Aditya Pakki <pakki001@umn.edu>
Link: https://lore.kernel.org/r/20191205231421.9333-1-pakki001@umn.eduSigned-off-by: Mark Brown <broonie@kernel.org>

drivers/spi/spi-dw.c

@@ -460,7 +460,8 @@ int dw_spi_add_host(struct device *dev, struct dw_spi *dws)
 	struct spi_controller *master;
 	int ret;
 
-	BUG_ON(dws == NULL);
+	if (!dws)
+		return -EINVAL;
 
 	master = spi_alloc_master(dev, 0);
 	if (!master)