UBUNTU: SAUCE: (upstream) netdev/fec: fix performance impact from mdio poll operation

BugLink: http://bugs.launchpad.net/bugs/546649
BugLink: http://bugs.launchpad.net/bugs/457878

After introducing phylib supporting, users experienced performace drop. That is
because of the mdio polling operation of phylib. Use msleep to replace the busy
waiting cpu_relax() and remove the warning message.

Signed-off-by: Bryan Wu <bryan.wu@canonical.com>
Acked-by: Andy Whitcroft <apw@canonical.com>
Signed-off-by: Andy Whitcroft <apw@canonical.com>