bcm63xx_enet: change the driver variables to static

bcm63xx_enetsw_driver and bcm63xx_enet_driver are only used in
bcm63xx_enet.c now, change them to static.

Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Link: https://lore.kernel.org/r/20220707135801.1483941-1-yangyingliang@huawei.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>