Add background MIB counter reading support.

Port MIB counters should only be read when there is link.  Otherwise it is
a waste of time as hardware never increases those counters.  There are
exceptions as some switches keep track of dropped counts no matter what.
Signed-off-by: NTristram Ha <Tristram.Ha@microchip.com>
Reviewed-by: NAndrew Lunn <andrew@lunn.ch>
Reviewed-by: NFlorian Fainelli <f.fainelli@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>