ath5k: Read and write the TSF via debugfs

This patch updates the ath5k specific entry in the debugfs to read and reset the TSF value, to allowing write it, too. This makes debugging the IBSS handling of wifi drivers _much_ easier.

Signed-off-by: Alina Friedrichsen <x-alina@gmx.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>