net: dsa: qca8k: move page cache to driver priv

There can be multiple qca8k switch on the same system. Move the static
qca8k_current_page to qca8k_priv and make it specific for each switch.

Signed-off-by: Ansuel Smith <ansuelsmth@gmail.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>