Revert "usb: gadget: allow to enable legacy drivers without USB_ETH"

This reverts commit 7a9618a2.

Romain Izard recently reported that commit 7a9618a2 ended up
allowing every legacy gadget driver to statically linked to the
kernel, however that doesn't work, since only one legacy gadget can be
bound to a controller. Because of that, let's revert the original commit
and fix the problem.

Reported-by: Romain Izard <romain.izard.pro@gmail.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>