net: mvpp2: fix GOP statistics loop start and stop conditions

GOP statistics from all ports of one instance of the driver are gathered
with one work recalled in loop in a workqueue. The loop is started when
a port is up, and stopped when a port is down. This last condition is
obviously wrong.

Fix this by having a work per port. This way, starting and stoping it
when the port is up or down will be fine, while minimizing unnecessary
CPU usage.

Fixes: 118d6298 ("net: mvpp2: add ethtool GOP statistics")
Reported-by: Stefan Chulski <stefanc@marvell.com>
Signed-off-by: Miquel Raynal <miquel.raynal@free-electrons.com>
Signed-off-by: David S. Miller <davem@davemloft.net>