spi: stm32: do not mandate cs_gpio

CS gpios is not mandatory, the driver should allow working
even when CS are not given.

Signed-off-by: Alain Volmat <alain.volmat@foss.st.com>
Link: https://lore.kernel.org/r/1612551572-495-3-git-send-email-alain.volmat@foss.st.com


Signed-off-by: Mark Brown <broonie@kernel.org>