usb: gadget: legacy gadgets are optional

With commit bc49d1d17dcf ("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: bc49d1d17dcf ("usb: gadget: don't couple configfs to legacy gadgets")
Cc: <[e-mail]> # 4.9+
Signed-off-by: Romain Izard <[e-mail]>
Signed-off-by: Felipe Balbi <[e-mail]>

drivers/usb/gadget/Kconfig

···
 choice
 	tristate "USB Gadget Drivers"
 	default USB_ETH
+	optional
 	help
 	  A Linux "Gadget Driver" talks to the USB Peripheral Controller
 	  driver through the abstract "gadget" API.  Some other operating