mt76: mt7615: check MT76_STATE_PM flag before accessing the device

Double-check if the device is in low-power state before accessing
registermap in mt7615_sta_rate_tbl_update() and in
mt7615_led_set_config()

Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>