net: macb: Fix ETH not found when clock not support

For the boards such as smartweb on which the clock driver isn't
supported, the ethernet fail to be found when booting up with
the below log.
---8<---
Net:   No ethernet found.
--->8---

Signed-off-by: Wenyou Yang <wenyou.yang@atmel.com>
Tested-by: Heiko Schocher <hs@denx.de>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>