iwlwifi: fix locking assertions

spin_is_locked() can return zero on some (UP?)
configurations because locks don't exist, and
that causes an endless amount of warnings. Use
lockdep_assert_held() instead, which has two
advantages:
 1) it verifies the current task is holding
    the lock or mutex
 2) it compiles away completely when lockdep
    is not enabled

Cc: stable@kernel.org [2.6.34+, maybe only parts of patch]
Reported-by: Thomas Meyer <thomas@m3y3r.de>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
diff --git a/drivers/net/wireless/iwlwifi/iwl-scan.c b/drivers/net/wireless/iwlwifi/iwl-scan.c
index b0c6b04..a4b3663 100644
--- a/drivers/net/wireless/iwlwifi/iwl-scan.c
+++ b/drivers/net/wireless/iwlwifi/iwl-scan.c
@@ -298,7 +298,7 @@
 
 static int iwl_scan_initiate(struct iwl_priv *priv, struct ieee80211_vif *vif)
 {
-	WARN_ON(!mutex_is_locked(&priv->mutex));
+	lockdep_assert_held(&priv->mutex);
 
 	IWL_DEBUG_INFO(priv, "Starting scan...\n");
 	set_bit(STATUS_SCANNING, &priv->status);