USB: EHCI: unlink multiple async QHs together

This patch (as1582) changes ehci-hcd's strategy for unlinking async
QHs.  Currently the driver never unlinks more than one QH at a time.
This can be inefficient and cause unnecessary delays, since a QH
cannot be reused while it is waiting to be unlinked.

The new strategy unlinks all the waiting QHs at once.  In practice the
improvement won't be very big, because it's somewhat uncommon to have
two or more QHs waiting to be unlinked at any time.  But it does
happen, and in any case, doing things this way makes more sense IMO.

The change requires the async unlinking code to be refactored
slightly.  Now in addition to the routines for starting and ending an
unlink, there are new routines for unlinking a single QH and starting
an IAA cycle.  This approach is needed because there are two separate
paths for unlinking async QHs:

	When a transfer error occurs or an URB is cancelled, the QH
	must be unlinked right away;

	When a QH has been idle sufficiently long, it is unlinked
	to avoid consuming DMA bandwidth uselessly.

In the first case we want the unlink to proceed as quickly as
possible, whereas in the second case we can afford to batch several
QHs together and unlink them all at once.  Hence the division of
labor.

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