mt76: mt7615: move mt7615_mcu_set_rates to mac.c

It bypasses the MCU, so it does not belong in mcu.c
Also make mt7615_mac_tx_rate_val static

Reviewed-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>