net: macb: Migrate to devm clock interface

Migrate to using the device managed interface for clocks and clean up
the associated error paths.

Signed-off-by: Soren Brinkmann <soren.brinkmann@xilinx.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>