net:macb: add line break

Without this patch we see annoying output like this:

---8<---
U-Boot> dhcp
macb1: PHY not foundmacb0: PHY present at 1
macb0: Starting autonegotiation...
--->8---

Signed-off-by: Andreas Bießmann <andreas.devel@googlemail.com>
cc: Joe Hershberger <joe.hershberger@gmail.com>