net: eth-uclass: Setup ROM source only when ROM reading passes

There is no reason to setup ROM source if read_rom_hwaddr hook doesn't
exist or reading mac address fails. It is ending up with confusion about
mac address source.

It is nicely visible if you put mac address to DT as
local-mac-address = [ff ff ff ff ff ff];
but also save ethaddr to variables
setenv -f ethaddr 02:18:31:7e:3e:01

Before this patch U-Boot prints that source is ROM
Address in ROM is		ff:ff:ff:ff:ff:ff
Address in environment is	02:18:31:7e:3e:01

After that source is DT:
Address in DT is		ff:ff:ff:ff:ff:ff
Address in environment is	02:18:31:7e:3e:01

Signed-off-by: Michal Simek <michal.simek@amd.com>