wifi: mt76: mt7996: rely on mt76_connac2_mac_tx_rate_val

In the Linux kernel, the following vulnerability has been resolved: 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.