mt76: mt7915: fix mib stats counter reporting to mac80211

In order to properly report MIB counters to mac80211, resets stats in
mt7915_get_stats routine() and hold mt76 mutex accessing MIB counters.
Sum up MIB counters in mt7915_mac_update_mib_stats routine.

Fixes: e57b7901 ("mt76: add mac80211 driver for MT7915 PCIe-based chipsets")
Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>