ath9k: remove unnecessary power save flags.

drv_config callback is called only after the ack for the nullframe
is received and so driver need not do anything special for this.

So remove NULLFUNC_COMPLETED, PS_ENABLED flags and bf_isnullfunc
flags from ath9k as mac80211 already handles them properly.

Signed-off-by: Senthil Balasubramanian <senthilkumar@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>