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: Adrian Bunk <bunk@stusta.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>