wifi: mt76: mt7915: reserve 8 bits for the index of rf registers

The value of regidx of rf registers is combined with WF selection and
offset. Extend the WF selection field from 4 to 8 bits since the
adie index should also be specified.

Signed-off-by: Evelyn Tsai <evelyn.tsai@mediatek.com>
Signed-off-by: Shayne Chen <shayne.chen@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>