netdev/fec: fix ifconfig eth0 down hang issue

BugLink: http://bugs.launchpad.net/bugs/559065

In fec open/close function, we need to use phy_connect and phy_disconnect
operation before we start/stop phy. Otherwise it will cause system hang.

Only call fec_enet_mii_probe() in open function, because the first open
action will cause NULL pointer error.

Signed-off-by: Bryan Wu <bryan.wu@canonical.com>
Acked-by: Tim Gardner <tim.gardner@canonical.com>
Acked-by: Colin King <colin.king@canonical.com>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>