mt76: mt7615: fix mib stats counter reporting to mac80211

[ Upstream commit 2eb6f6c4 ]

In order to properly report MIB counters to mac80211, resets stats in
mt7615_get_stats routine and hold mt76 mutex accessing MIB counters.
Sum up MIB counters in mt7615_mac_update_mib_stats routine.

Fixes: c388d858 ("mt76: mt7615: add a get_stats() callback")
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>
Signed-off-by: Sasha Levin <sashal@kernel.org>