Clear SPI TX FIFO

There's no easy mechanism to clear TXFIFO. Force resetting the peripheral and reinitializing is reported to work on the F0 and other families. On the H7 family, resetting SPE clears the FIFO, but not here, per the RM.

https://community.st.com/s/question/0D50X00009XkeT8/stm32f0-spi-txfifo-flush

HAL_SPIEx_FlushRxFifo just reads out DR until it's empty, no special command or anything.

https://github.com/STMicroelectronics/STM32CubeG0/blob/5cb06333a6a43cefbe145f10a5aa98d3cc4cffee/Drivers/STM32G0xx_HAL_Driver/Src/stm32g0xx_hal_spi_ex.c#L80

You need to use the peripheral reset control bit from RCC/SYS and reconfigure the spi...

The HAL lets you do the RCC/SYS reset through some macros:

HAL_SPI_Abort(SPIxTypedef);
 
__HAL_RCC_SPI2_FORCE_RESET();
 
__HAL_RCC_SPI2_RELEASE_RESET();