be2net: use mutex instead of spin lock for mbox_lock

Since the mbox polling uses the schedule_timeout, the mbox_lock should be
a mutex and not a spin lock.
The commit f25b03a replaced udelay() with schedule_timeout() but didn't
change mbox_lock to semaphore or mutex.

Signed-off-by: Ivan Vecera <ivecera@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/drivers/net/benet/be_main.c b/drivers/net/benet/be_main.c
index 93354ee..fd251b5 100644
--- a/drivers/net/benet/be_main.c
+++ b/drivers/net/benet/be_main.c
@@ -2677,7 +2677,7 @@
 	}
 	memset(mc_cmd_mem->va, 0, mc_cmd_mem->size);
 
-	spin_lock_init(&adapter->mbox_lock);
+	mutex_init(&adapter->mbox_lock);
 	spin_lock_init(&adapter->mcc_lock);
 	spin_lock_init(&adapter->mcc_cq_lock);