ehci: remove duplicate debug_async_open() prototype in ehci-dbg.c

This patch removes the duplicate of debug_async_open() prototype following
three lines below the debug_async_open() declaration.

Signed-off-by: Xenia Ragiadakou <burzalodowa@gmail.com>
Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>