mt76: mt7615: do not update pm stats in case of error

Do not update pm stats if mt7615_mcu_fw_pmctrl returns an error.

Fixes: abe912ae ("mt76: mt7663: add awake and doze time accounting")
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>