mac80211: fix deadlock with multiple interfaces

The locking around ieee80211_recalc_smps is
buggy -- it cannot acquire another interface's
mutex while the iflist mutex is held because
another code path could be holding the iface
mutex and trying to acquire the iflist mutex.

But the locking is also unnecessary, we only
check "ifmgd->associated" as a bool, and don't
use the pointer (in check_mgd_smps).

Reported-by: Ben Greear <greearb@candelatech.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>