mt76: mt7921: do not update pm states in case of error

Do not update pm stats if mt7921e_mcu_fw_pmctrl routine returns an
error.

Fixes: 36873246 ("mt76: mt7921: add awake and doze time accounting")
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>