Don't call em_stop() from the watchdog since it requires the controller

mutex to be locked.  It is redundant since em_init() is called and this
correctly locks the mutex and calls em_stop().

5.2 release candidate since this can cause a panic if the watchdog
expires.

Tested by:	kuriyama

sys/dev/em/if_em.c

@@ -768,7 +768,6 @@ em_watchdog(struct ifnet *ifp)
 
 	ifp->if_flags &= ~IFF_RUNNING;
 
-	em_stop(adapter);
 	em_init(adapter);
 
 	ifp->if_oerrors++;