netdev: bfin_mac: handle timeouts with the MDIO registers gracefully

Have the low level MDIO functions pass back up timeout information so we
don't waste time polling them multiple times when there is a problem, and
so we don't let higher layers think the device is available when it isn't.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: David S. Miller <davem@davemloft.net>