mt76: mt7615: fix possible memory leak in mt7615_tm_set_tx_power

Fix a memory leak in mt7615_tm_set_tx_power routine if
mt7615_eeprom_get_target_power_index fails.
Moreover do not account req_header twice in mcu skb allocation.

Fixes: 4f0bce1c ("mt76: mt7615: implement testmode support")
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>