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

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

This reverts commit e789ad6d2ace243bdd34f0d6bcec01f35e624ccb.

Romain Izard recently reported that commit e789ad6d2ace 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>