ath9k: fix power save race conditions

ath9k: fix power save race conditions

ath9k has a race on putting the chip into network sleep and
having registers read from hardware. The race occurs because
although ath9k_ps_restore() locks its own callers it makes use
of some variables which get altered in the driver at different
code paths. The variables are the ps_enabled and ps_flags.

This is easily reprodicible in large network environments when
roaming with the wpa_supplicant simple bgscan. You'd get some
0xdeadbeef read out on certain registers such as:

ath: timeout (100000 us) on reg 0x806c: 0xdeadbeef & 0x01f00000 != 0x00000000
ath: RX failed to go idle in 10 ms RXSM=0xdeadbeef

ath: timeout (100000 us) on reg 0x7000: 0xdeadbeef & 0x00000003 != 0x00000000
ath: Chip reset failed

The fix is to protect the ath9k_config(hw, IEEE80211_CONF_CHANGE_PS)
calls with a spin_lock_irqsave() which will disable contendors for
these variables from interrupt context, timers, re-entry from mac80211
on the same callback, and most importantly from ath9k_ps_restore()
which is the only call which will put the device into network sleep.

There are quite a few threads and bug reports on these a few of them are:

https://bugs.launchpad.net/ubuntu/karmic/+source/linux/+bug/407040
http://code.google.com/p/chromium-os/issues/detail?id=5709
http://code.google.com/p/chromium-os/issues/detail?id=5943

Stable fixes apply to [2.6.32+]

Cc: stable@kernel.org
Cc: Paul Stewart <pstew@google.com>
Cc: Amod Bodas <amod.bodas@atheros.com>
Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>