wifi: mt76: mt7921: rename mt7921_vif in mt792x_vif

This is a preliminary patch to introduce WiFi7 chipset support

Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Deren Wu <deren.wu@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>