MIPS: ralink: dts: gardena_smart_gateway_mt7688: Limit UART1

The radio module asserts CTS when its RX buffer has 10 bytes left.
Putting just 8 instead of 16 bytes into the UART1 TX buffer on the Linux
side ensures to not overflow the RX buffer on the radio module side.

Signed-off-by: Reto Schneider <reto.schneider@husqvarnagroup.com>
Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Paul Burton <paul.burton@mips.com>
Signed-off-by: Paul Burton <paulburton@kernel.org>
Cc: linux-mips@vger.kernel.org