net: dsa: qca8k: move regmap init in probe and set it mandatory

In preparation for regmap conversion, move regmap init in the probe
function and make it mandatory as any read/write/rmw operation will be
converted to regmap API.

Signed-off-by: Ansuel Smith <ansuelsmth@gmail.com>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>