net: cosmetic: Make the MAC address string less magical

In u-boot printf has been extended with the %pM formatter to allow
printing of MAC addresses. However buffers that want to store a MAC
address cannot safely get the size. Add a define for this case so the
string of a MAC address can be reliably obtained.

Signed-off-by: Olliver Schinagl <oliver@schinagl.nl>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>