spi: stm32: Fix comments compilation warnings

Fix all functions and structure descriptions to have the driver
warning free when built with W=1.

Signed-off-by: Alain Volmat <alain.volmat@st.com>
Reviewed-by: Amelie Delaunay <amelie.delaunay@st.com>
Link: https://lore.kernel.org/r/1584711857-9162-1-git-send-email-alain.volmat@st.com


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