spi: stm32: make SPI_MASTER_MUST_TX flags only specific to STM32F4

spi: stm32: make SPI_MASTER_MUST_TX flags only specific to STM32F4

Commit 77e334858c37 ("spi: stm32: Add 'SPI_SIMPLEX_RX', 'SPI_3WIRE_RX' support for stm32f4")
allowed to properly communicate with the st-gyro-spi even when
there is no tx_buf provided by setting the flag SPI_MASTER_MUST_TX and
thus forcing a dummy TX buffer to work in Full Duplex.
This behavior should kept only for the STM32F4 and not for other
compatible since the STM32H7 do support SIMPLEX_RX and SIMPLEX_TX.
Add the flags variable within the struct stm32_spi_cfg so that flags
used at master registration time are compatible specific.

Fixes: 77e334858c37 ("spi: stm32: Add 'SPI_SIMPLEX_RX', 'SPI_3WIRE_RX' support for stm32f4")
Signed-off-by: Alain Volmat <alain.volmat@foss.st.com>
Link: https://lore.kernel.org/r/20220119093245.624878-3-alain.volmat@foss.st.com
Signed-off-by: Mark Brown <broonie@kernel.org>