netdev/fec: fix compiling warning

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

Missed to set .ndo_do_ioctl as fec_enet_ioctl, so will generate a compiling
warning due to nobody uses fec_enet_ioctl. This patch fixed that.

Signed-off-by: Bryan Wu <bryan.wu@canonical.com>
Acked-by: Amit Kucheria <amit.kucheria@canonical.com>
Acked-by: Colin King <colin.king@canonical.com>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>