serial: stm32: correct loop for dma error handling

In this error handling, "transmit_chars_dma" function will call
"transmit_chars_pio" once per characters. But "transmit_chars_pio" will
continue to send characters while xmit buffer is not empty.

Remove this useless loop, one call is sufficient.

Signed-off-by: Valentin Caron <valentin.caron@foss.st.com>
Link: https://lore.kernel.org/r/20220104182445.4195-5-valentin.caron@foss.st.com


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