The AT91SAM9G25-based GARDENA smart Gateway uses USART3 (ttyS1) to
communicate (over PPP, at 115200 baud) with the radio module.

We have been experiencing packet loss and seeing errors on the ppp0
interface. Enabling DMA for the serial interface resolves the issues.
Co-developed-by: Michael Zimmermann <michael.zimmermann@grandcentrix.net>
Signed-off-by: Michael Zimmermann <michael.zimmermann@grandcentrix.net>
Signed-off-by: Ezra Buehler <ezra.buehler@husqvarnagroup.com>
Acked-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Link: https://lore.kernel.org/r/20240102161839.702625-3-ezra.buehler@husqvarna.com
[claudiu.beznea: s/Co-Developed-by/Co-developed-by to make checkpatch.pl
 happy, s/at91/microchip in commit title]
Signed-off-by: Claudiu Beznea <claudiu.beznea@tuxon.dev>