net: marvell: prestera: Add missing of_node_put() in prestera_switch_set_base_mac_addr

This node pointer is returned by of_find_compatible_node() with
refcount incremented. Calling of_node_put() to aovid the refcount leak.

Fixes: 501ef306

 ("net: marvell: prestera: Add driver for Prestera family ASIC devices")
Signed-off-by: Miaoqian Lin <linmq006@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>