ath6kl: Add debugfs file entry to dump credit distribution stats

It would be at <dbgfs_root>/ieee80211/phyX/ath6kl/credit_dist_stats.

Signed-off-by: Vasanthakumar Thiagarajan <vthiagar@qca.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>