ath: ath9k: use debugfs_create_devm_seqfile() helper for seq_file entries

Use the helper to get rid of the file operations per debugfs file. The
device driver data contains struct ieee80211_hw pointer and the
struct ath9k_softc pointer is assigned to ieee80211_hw::priv so it can
be accessed in the seq_file read operation.

Cc: ath9k-devel@lists.ath9k.org
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>