libertas: use spin_is_locked() instead of spin_trylock() in lbs_interrupt()

We get scary warnings on UP if we use spin_trylock() and find, as we
hoped, that the lock in question is already locked.
Signed-off-by: David Woodhouse <dwmw2@infradead.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

drivers/net/wireless/libertas/main.c

@@ -1414,8 +1414,7 @@ void lbs_interrupt(struct lbs_private *priv)
 
 	lbs_deb_thread("lbs_interrupt: intcounter=%d\n", priv->intcounter);
 
-	if (spin_trylock(&priv->driver_lock)) {
-		spin_unlock(&priv->driver_lock);
+	if (!spin_is_locked(&priv->driver_lock)) {
 		printk(KERN_CRIT "%s called without driver_lock held\n", __func__);
 		WARN_ON(1);
 	}