usb: gadget: USB_ETHER requires network support

In order to compile the USB Ethernet gadget support we require that NET
is enabled, add that dependency here.

Signed-off-by: Alex Kiernan <alex.kiernan@gmail.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>

drivers/usb/gadget/Kconfig

@@ -158,6 +158,7 @@ endif # USB_GADGET_DOWNLOAD
 
 config USB_ETHER
 	bool "USB Ethernet Gadget"
+	depends on NET
 	default y if ARCH_SUNXI && USB_MUSB_GADGET
 	help
 	  Creates an Ethernet network device through a USB peripheral