ath10k: Implement sta_add_debugfs

Add per station debugfs files when a station is added to mac80211's
station list. This helps to group peer specific debugfs entries
altogether. Right now this callback adds support to test aggregation
procedures (addba/addba_resp/delba) manually.

To enable automatic aggregation in target,
echo 0 >/sys/kernel/debug/ieee80211/phyX/netdev:wlanX/
	stations/XX:XX:XX:XX:XX:XX/aggr_mode

For manual mode,
echo 1 >/sys/kernel/debug/ieee80211/phyX/netdev:wlanX/
	stations/XX:XX:XX:XX:XX:XX/aggr_mode

Signed-off-by: Rajkumar Manoharan <rmanohar@qti.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>