USB: add missing kerneldoc line for "needs_binding"

This patch (as1117) adds a kerneldoc line for the "needs_binding"
field in struct usb_interface.  It was accidentally omitted when the
field was added.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>