ath: use seq_file api for ath9k debugfs files

The debugfs files that are defined in debug.c which are read-only
and using a simple_open as .open file operation have been modified
to use the single_open seq_file API. This simplifies the read
functions defining the file contents.

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