serial: stm32: use DMAT as a configuration bit

DMAT is a configuration bit so it should be set at the startup of uart
port and not when a DMA transfer begins.

This patch move set of DMAT into set_termios and remove DMAT reset except
in shutdown.

Signed-off-by: Valentin Caron <valentin.caron@foss.st.com>
Link: https://lore.kernel.org/r/20230808161906.178996-3-valentin.caron@foss.st.com


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