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>