rockchip: misc: don't fail if eth_addr already set

rockchip_setup_macaddr() runs from an initcall, so returning an error
code will make that initcall fail thus breaking the boot process.

And if an ethernet address is already set this is definitly not a
cause for that, so just return success in that case.

Fixes: 04825384 ("rockchip: rk3399: derive ethaddr from cpuid");
Signed-off-by: Heiko Stuebner <heiko.stuebner@theobroma-systems.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>