net: dsa: Add __percpu property to prevent warnings

net/dsa/slave.c:505:13: warning: incorrect type in initializer (different address spaces)
net/dsa/slave.c:505:13:    expected void const [noderef] <asn:3> *__vpp_verify
net/dsa/slave.c:505:13:    got struct pcpu_sw_netstats *

Add the needed _percpu property to prevent this warning.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>