Wait for the link to come up on kirkwood network init

This patch makes the device wait for up to 5 seconds for the link to
come up, similar to what many of the other network drivers do. This
avoids confusing situations where, e.g., a tftp fails when initiated
early after U-boot has started (before the link has come up).

Signed-off-by: Simon Kagstrom <simon.kagstrom@netinsight.net>
Signed-off-by: Ben Warren <biggerbadderben@gmail.com>