mt76: mt7915: add .sta_add_debugfs support

This generation supports much more per-peer statistics than legacy ones,
so add .sta_add_debugfs accordingly.

This is convenient to set/get more settings/counters in the long run.

Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>