usb: gadget: Fix g_ether interface link status

A "usb0" interface that has never been connected to a host has an unknown
operstate, and therefore the IFF_RUNNING flag is (incorrectly) asserted
when queried by ifconfig, ifplugd, etc.  This is a result of calling
netif_carrier_off() too early in the probe function; it should be called
after register_netdev().

Similar problems have been fixed in many other drivers, e.g.:

    e826eafa (bonding: Call netif_carrier_off after register_netdevice)
    0d672e9f (drivers/net: Call netif_carrier_off at the end of the probe)
    6a3c869a (cxgb4: fix reported state of interfaces without link)

Fix is to move netif_carrier_off() to the end of the function.

Cc: stable@vger.kernel.org
Signed-off-by: Kevin Cernekee <cernekee@gmail.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>