mt76: mt7915: fix aggr len debugfs node

Similar to mt7921, fix 802.11 aggr len debugfs reporting for mt7915 driver.

Fixes: e57b7901

 ("mt76: add mac80211 driver for MT7915 PCIe-based chipsets")
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>