usb: gadget: u_ether: Remove warning from gether_disconnect()

gether_disconnect() is always called before gether_connect() by the different
USB ethernet functions, so this warning was firing during normal operation.

Change-Id: I178cc7d1f67e9e87ac2e99c6674c4db8fcddf4a0
Signed-off-by: Mike Lockwood <lockwood@android.com>