wifi: mt76: mt7921: fix the wrong rate pickup for the chanctx driver

The variable band should be determined by the ieee80211_chanctx_conf when
the driver is a kind of chanctx one e.g mt7921 and mt7922 driver so we
added the extension to mt76_connac2_mac_tx_rate_val by distinguishing if
it can support chanctx to fix the incorrect rate pickup.

Fixes: 41ac53c8 ("wifi: mt76: mt7921: introduce chanctx support")
Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Reviewed-by: David Ruth <druth@chromium.org>
Tested-by: David Ruth <druth@chromium.org>
Signed-off-by: Deren Wu <deren.wu@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>