wifi: mt76: sdio: poll sta stat when device transmits data

It is not meaningful to poll sta stat when there is no data traffic.
So polling sta stat when the device has transmitted data instead to save
CPU power.

That implies that it is unallowed the stat_work to work while MCU is being
initialized in the really early stage to fix the possible time to time MCU
initialization failure.

Fixes: d39b52e3

 ("mt76: introduce mt76_sdio module")
Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>