wifi: mt76: mt7996: rely on mt76_connac2_mac_tx_rate_val

In order to fix a possible NULL pointer dereference in
mt7996_mac_write_txwi() of vif pointer, export
mt76_connac2_mac_tx_rate_val utility routine and reuse it
in mt7996 driver.

Fixes: 98686cd2 ("wifi: mt76: mt7996: add driver for MediaTek Wi-Fi 7 (802.11be) devices")
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>