spi: soft_spi: Fix data abort if slave is not probed

In case spi_get_bus_and_cs callback is used, spi bus is first probed
then slave devices are probed. To avoid a data abort in soft_spi probe
function, we need to check that (slave != NULL).

If slave is NULL, cs_flags and clk_flags will be initialized with
respectively GPIOD_ACTIVE_LOW and 0.

Signed-off-by: Christophe Kerello <christophe.kerello@st.com>
Signed-off-by: Patrice Chotard <patrice.chotard@st.com>
Reviewed-by: Jagan Teki <jagan@amarulasolutions.com>
diff --git a/drivers/spi/soft_spi.c b/drivers/spi/soft_spi.c
index b06883f..b80f810 100644
--- a/drivers/spi/soft_spi.c
+++ b/drivers/spi/soft_spi.c
@@ -215,8 +215,8 @@
 	int cs_flags, clk_flags;
 	int ret;
 
-	cs_flags = (slave->mode & SPI_CS_HIGH) ? 0 : GPIOD_ACTIVE_LOW;
-	clk_flags = (slave->mode & SPI_CPOL) ? GPIOD_ACTIVE_LOW : 0;
+	cs_flags = (slave && slave->mode & SPI_CS_HIGH) ? 0 : GPIOD_ACTIVE_LOW;
+	clk_flags = (slave && slave->mode & SPI_CPOL) ? GPIOD_ACTIVE_LOW : 0;
 
 	if (gpio_request_by_name(dev, "cs-gpios", 0, &plat->cs,
 				 GPIOD_IS_OUT | cs_flags) ||