spi: stm32: make SPI_MASTER_MUST_TX flags only specific to STM32F4

Commit 61367d0 ("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: 61367d0 ("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>

drivers/spi/spi-stm32.c

@@ -231,6 +231,7 @@ struct stm32_spi;
  * @baud_rate_div_min: minimum baud rate divisor
  * @baud_rate_div_max: maximum baud rate divisor
  * @has_fifo: boolean to know if fifo is used for driver
+ * @flags: compatible specific SPI controller flags used at registration time
  */
 struct stm32_spi_cfg {
 	const struct stm32_spi_regspec *regs;
@@ -251,6 +252,7 @@ struct stm32_spi_cfg {
 	unsigned int baud_rate_div_min;
 	unsigned int baud_rate_div_max;
 	bool has_fifo;
+	u16 flags;
 };
 
 /**
@@ -1720,6 +1722,7 @@ static const struct stm32_spi_cfg stm32f4_spi_cfg = {
 	.baud_rate_div_min = STM32F4_SPI_BR_DIV_MIN,
 	.baud_rate_div_max = STM32F4_SPI_BR_DIV_MAX,
 	.has_fifo = false,
+	.flags = SPI_MASTER_MUST_TX,
 };
 
 static const struct stm32_spi_cfg stm32h7_spi_cfg = {
@@ -1852,7 +1855,7 @@ static int stm32_spi_probe(struct platform_device *pdev)
 	master->prepare_message = stm32_spi_prepare_msg;
 	master->transfer_one = stm32_spi_transfer_one;
 	master->unprepare_message = stm32_spi_unprepare_msg;
-	master->flags = SPI_MASTER_MUST_TX;
+	master->flags = spi->cfg->flags;
 
 	spi->dma_tx = dma_request_chan(spi->dev, "tx");
 	if (IS_ERR(spi->dma_tx)) {