net: mv643xx_eth: update statistics timer from timer context only

Each port driver installs a periodic timer to update port statistics
by calling mib_counters_update. As mib_counters_update is also called
from non-timer context, we should not reschedule the timer there but
rather move it to timer-only context.

Signed-off-by: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>
Acked-by: Jason Cooper <jason@lakedaemon.net>
Signed-off-by: David S. Miller <davem@davemloft.net>