[][MAC80211][wifi6][MT76][Add debugfs knob to dump TRx drop]

[Description]
Add debugfs knob to dump TRx drop in MT76.
Usage:
Dump Tx drop: cat /sys/kernel/debug/ieee80211/phyX/mt76/tx_drop
Dump Rx drop: cat /sys/kernel/debug/ieee80211/phyX/mt76/rx_drop
Reset counter: echo 0 > wlan_idx; echo 1 > reset_counter

[Release-log]
N/A

Change-Id: I07ea8f29ed1735b98b36d59efcba19949f7f03e7
Reviewed-on: https://gerrit.mediatek.inc/c/openwrt/feeds/mtk_openwrt_feeds/+/8825269
3 files changed