mt76: mt7915: optimize mt7915_mac_sta_poll

Since DMA completion does not imply tx completion, it makes more sense to
poll for airtime from mt7915_mac_tx_free.
Reduce the runtime of the function by moving all items from dev->sta_poll_list
to a local list once and process any stations that were added afterwards
on the next run

Signed-off-by: Felix Fietkau <nbd@nbd.name>