USB: EHCI: add pointer to end of async-unlink list

This patch (as1570) adds a pointer for the end of ehci-hcd's
async-unlink list.  The list (which is actually a queue) is singly
linked, so having a pointer to its end makes adding new entries easier
-- there's no longer any need to scan through the whole list.

In principle it could be changed to a standard doubly-linked list.  It
turns out that doing so actually makes the code less clear, so I'm
leaving it as is.

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