wifi: mt76: mt7996: set DMA mask to 36 bits for boards with more than 4GB of RAM

Introduce the capability to run mt7996 driver on boards with more than
4GB of memory.

Co-developed-by: Rex Lu <rex.lu@mediatek.com>
Signed-off-by: Rex Lu <rex.lu@mediatek.com>
Signed-off-by: Sujuan Chen <sujuan.chen@mediatek.com>
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>