serial: stm32: fix flow control transfer in DMA mode

If flow control is enabled, framework will call stop_tx to
pause transfer and then call start_tx to resume transfer.

Clear USART_CR3_DMAT bit in stop_tx ops to pause DMA transfer.

Signed-off-by: Erwan Le Ray <erwan.leray@foss.st.com>
Signed-off-by: Valentin Caron <valentin.caron@foss.st.com>
Link: https://lore.kernel.org/r/20220104182445.4195-4-valentin.caron@foss.st.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>