serial: stm32: call stm32_transmit_chars locked

stm32_transmit_chars should be called under lock also in tx DMA callback.

Fixes: 34891872 ("serial: stm32: adding dma support")
Signed-off-by: Erwan Le Ray <erwan.leray@foss.st.com>
Signed-off-by: Fabrice Gasnier <fabrice.gasnier@foss.st.com>
Link: https://lore.kernel.org/r/20210304162308.8984-10-erwan.leray@foss.st.com


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