net/vmxnet3: retain counters on restart

net/vmxnet3: retain counters on restart

Most NICs like virtio, igb/ixgbe etc. don't reset counters on
dev_start and arguably this helps in monitoring the counters
across a longer time span with multiple device start/stops.
vmxnet3 behavior is opposite to that and counters are reset by
the host side implementation each time the device is restarted.

Change the driver to save the counters in its private context
before it is reset by writing CMD_ACTIVATE to REG_CMD.

Signed-off-by: Nachiketa Prachanda <nprachan@brocade.com>
Acked-by: Shrikrishna Khare <skhare@vmware.com>