net: dsa: b53: relax is63xx() condition

BCM63xx switches are present on bcm63xx and bmips devices.

Signed-off-by: Álvaro Fernández Rojas <noltari@gmail.com>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>