mt76x0: use mt76x02_mac_work as stats handler

Use mt76x02_mac_work utility routine as stats workqueue handler
and remove duplicated code. Moreover run mac stuck check in
mt76x02_mac_work for client interfaces.
Remove no longer used avg_ampdu_len and mt76x02_mac_stats data
structure in mt76x02_dev

Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi@redhat.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>