serial: atmel: don't stop the transmitter when doing PIO

Writing ATMEL_US_TXDIS to ATMEL_US_CR makes the transmitter NOT to send
the just queued character. This means when the character is last and
uart calls ops->stop_tx(), the character is not sent at all.

The usart datasheet is not much specific on this, it just says the
transmitter is stopped. But apparently, the character is dropped. So
we should stop the transmitter only for DMA and PDC transfers to not
send any more characters. For PIO, this is unexpected and deviates from
other drivers. In particular, the below referenced commit broke TX as it
added a call to ->stop_tx() after the very last character written to the
transmitter.

So fix this by limiting the write of ATMEL_US_TXDIS to DMA transfers
only.

Even there, I don't know if it is correctly implemented. Are all the
queued characters sent once ->start_tx() is called? Anyone tested flow
control -- be it hard (RTSCTS) or the soft (XOFF/XON) one?

Fixes: 2d141e68

 ("tty: serial: use uart_port_tx() helper")
Cc: Richard Genoud <richard.genoud@gmail.com>
Cc: Nicolas Ferre <nicolas.ferre@microchip.com>
Cc: Alexandre Belloni <alexandre.belloni@bootlin.com>
Cc: Claudiu Beznea <claudiu.beznea@microchip.com>
Cc: linux-arm-kernel@lists.infradead.org
Reported-by: Michael Walle <michael@walle.cc>
Signed-off-by: Jiri Slaby (SUSE) <jirislaby@kernel.org>
Link: https://lore.kernel.org/r/20221123082736.24566-2-jirislaby@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>