spi: stm32: remove inexistant variables in struct stm32_spi_cfg comment

Variables 'can_dma' and 'has_startbit' are described within the
struct stm32_spi_cfg comment but have never existed in this structure
so remove them.

Signed-off-by: Alain Volmat <alain.volmat@foss.st.com>
Link: https://lore.kernel.org/r/20220119093245.624878-2-alain.volmat@foss.st.com


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