usb: gadget: legacy gadgets are optional

With commit bc49d1d1 ("usb: gadget: don't couple configfs to legacy
gadgets"),it is possible to build a modular kernel with both built-in
configfs support and modular legacy gadget drivers.

But when building a kernel without modules, it is also necessary to be
able to build with configfs but without any legacy gadget driver. This
was a possible configuration when the USB_CONFIGFS was a part of the
choice options, but not anymore.

Mark the choice for legacy gadget drivers as optional restores this.

Fixes: bc49d1d1 ("usb: gadget: don't couple configfs to legacy gadgets")
Cc: <stable@vger.kernel.org> # 4.9+
Signed-off-by: Romain Izard <romain.izard.pro@gmail.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>