of_net: Fix missing of_find_device_by_node ref count drop

of_find_device_by_node takes a reference to the embedded struct device
which needs to be dropped after use.

Fixes: d01f449c ("of_net: add NVMEM support to of_get_mac_address")
Reported-by: kbuild test robot <lkp@intel.com>
Reported-by: Julia Lawall <julia.lawall@lip6.fr>
Signed-off-by: Petr Štetiar <ynezz@true.cz>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>