usb gadget: fix ethernet link reports to ethtool

The g_ether USB gadget driver currently decides whether or not there's a
link to report back for eth_get_link based on if the USB link speed is
set. The USB gadget speed is however often set even before the device is
enumerated. It seems more sensible to only report a "link" if we're
actually connected to a host that wants to talk to us. The patch below
does this for me - tested with the PXA27x UDC driver.

Signed-off-by: Jonathan McDowell <noodles@earth.li>
Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Cc: stable <stable@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>