ath9k: Fix warnings on card removal

ath9k: Fix warnings on card removal

The recently added warning message on power change failure
is not needed on device removal.

ath: Failed to wakeup in 500us
------------[ cut here ]------------
WARNING: at drivers/net/wireless/ath/ath9k/hw.c:1618
ath9k_hw_setpower+0x61f/0x630 [ath9k_hw]()
Hardware name: 64756D6
Pid: 540, comm: kworker/u:3 Not tainted 2.6.37-rc6-wl #37
Call Trace:
 [<ffffffff810501aa>] warn_slowpath_common+0x7a/0xb0
 [<ffffffffa056e280>] ? ath9k_iowrite32+0x0/0x90 [ath9k]
 [<ffffffff810501f5>] warn_slowpath_null+0x15/0x20
 [<ffffffffa05226ef>] ath9k_hw_setpower+0x61f/0x630 [ath9k_hw]
 [<ffffffffa05700e5>] ath9k_ps_wakeup+0x85/0xd0 [ath9k]
 [<ffffffffa0570685>] ath9k_configure_filter+0x25/0x80 [ath9k]
 [<ffffffffa04dde43>] ieee80211_configure_filter+0x133/0x190 [mac80211]
 [<ffffffffa04ee502>] ieee80211_do_stop+0x132/0x540 [mac80211]
 [<ffffffff813466ff>] ? _raw_spin_unlock_bh+0x1f/0x30
 [<ffffffff812b6923>] ? dev_deactivate+0x1c3/0x1e0
 [<ffffffffa04ee925>] ieee80211_stop+0x15/0x20 [mac80211]
 [<ffffffff8129d1b6>] __dev_close+0x56/0x90

Signed-off-by: Rajkumar Manoharan <rmanoharan@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>