serial: atmel: cleanup atmel_start+stop_tx()

Define local variables holding information about whether pdc or dma is
used in the HW. These are retested several times by calls to
atmel_use_pdc_tx() and atmel_use_dma_tx(). So to make the code more
readable, simply cache the values.

This is also a preparatory patch for the next one (where is_pdc is used
once more in atmel_stop_tx()).

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/20221123082736.24566-1-jirislaby@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>