net: Move initialization of Au1x00 SoC ethernet MAC to cpu_eth_init

This patch will move au1x00_eth_initialize from net/eth.c to cpu_eth_init
as a part of ongoing eth_initialize cleanup work.  The function ret value
is also fixed as it should be negative on fail.

Signed-off-by: Shinya Kuribayashi <skuribay@ruby.dti.ne.jp>
Signed-off-by: Ben Warren <biggerbadderben@gmail.com>