USB: fix the USB_GADGET_DUMMY_HCD dependencies

If USB=m, USB_GADGET=y, the option USB_GADGET_DUMMY_HCD mustn't be 
offered since selecting it results in a compile error.

This patch fixes kernel Bugzilla #6534 reported by Toralf Frster.
Signed-off-by: NAdrian Bunk <bunk@stusta.de>
Signed-off-by: NGreg Kroah-Hartman <gregkh@suse.de>

drivers/usb/gadget/Kconfig

@@ -207,7 +207,7 @@ config USB_AT91
 
 config USB_GADGET_DUMMY_HCD
 	boolean "Dummy HCD (DEVELOPMENT)"
-	depends on USB && EXPERIMENTAL
+	depends on (USB=y || (USB=m && USB_GADGET=m)) && EXPERIMENTAL
 	select USB_GADGET_DUALSPEED
 	help
 	  This host controller driver emulates USB, looping all data transfer