mlxsw: Do not pass around driver_priv directly

Instead of that, pass mlxsw_core and use a helper to get driver priv
from driver code. Looks much cleaner that way.

Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Reviewed-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>