stm32: Clear SPE flag on a change to SPI CR1 register

The stm32 specs indicate that the SPE bit must be cleared before
changing the CPHA or CPOL bits.

Reported by @cbc02009 and @bigtreetech.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

src/stm32/spi.c

@@ -100,6 +100,12 @@ void
 spi_prepare(struct spi_config config)
 {
     SPI_TypeDef *spi = config.spi;
+    uint32_t cr1 = spi->CR1;
+    if (cr1 == config.spi_cr1)
+        return;
+    // The SPE bit must be disabled before changing CPOL/CPHA bits
+    spi->CR1 = cr1 & ~SPI_CR1_SPE;
+    spi->CR1; // Force flush of previous write
     spi->CR1 = config.spi_cr1;
 }