net: macb: Use devm_ioremap()

Use the device managed version of ioremap to remap IO memory,
simplifying 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>