mt76: mt7915: introduce band_idx in mt7915_phy

The wfsys of MT7986 has only single adie chip for non-dbdc devices,
and it binds to band1 by default. Hence this patch adds band_idx to
explicitly configure phy accordingly.

Co-developed-by: Sujuan Chen <sujuan.chen@mediatek.com>
Signed-off-by: Sujuan Chen <sujuan.chen@mediatek.com>
Co-developed-by: Shayne Chen <shayne.chen@mediatek.com>
Signed-off-by: Shayne Chen <shayne.chen@mediatek.com>
Signed-off-by: Bo Jiao <Bo.Jiao@mediatek.com>
Reviewed-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: Bo Jiao <Bo.Jiao@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>