usb: gadget: allow to enable legacy drivers without USB_ETH

Considerable time ago the legacy gadget menu was added inside the
USB_ETH choice. I think this was a mistake and that the legacy
gadget menu should have been added after "endchoice" instead of
before. Hence this patch.

Fixes: commit 8443f2d2

 ("usb: gadget: Gadget directory cleanup - group legacy gadgets")
Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>
Reviewed-by: Hannes Reinecke <hare@suse.com>
Cc: Nicholas Bellinger <nab@linux-iscsi.org>
Cc: Andrzej Pietrasiewicz <andrzej.p@samsung.com>
Cc: linux-usb@vger.kernel.org
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>