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
struct ath9k_softc pointer is set as device driver data to be obtained
in the seq_file read operation.

Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>