tty: atmel_serial: use single DMA mapping for TX

tty: atmel_serial: use single DMA mapping for TX

dma_map_single() provides much easier interface for simple mappings as
used for TX in atmel_serial. So switch to that, removing all the s-g
unnecessary handling.

Note that it is not easy (maybe impossible) to use kfifo_dma_* API for
atmel's serial purposes. It handles DMA very specially.

Signed-off-by: Jiri Slaby (SUSE) <jirislaby@kernel.org>
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@tuxon.dev>
Cc: linux-arm-kernel@lists.infradead.org
Link: https://lore.kernel.org/r/20240405060826.2521-14-jirislaby@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>