Revert "ehci: don't flush cache on doorbell rings."

Revert "ehci: don't flush cache on doorbell rings."

This reverts commit 9bc3a3a216e2689bfcdd36c3e079333bbdbf3ba0, which got
added to fix an issue where the real, underlying cause was not stopping
the ep queue on an error.

Now that the underlying cause is fixed by the "usb: Halt ep queue and
cancel pending packets on a packet error" patch, the "don't flush" fix
is no longer needed.

Not only is it not needed, it causes us to see cancellations (unlinks)
done by the Linux EHCI driver too late, which in combination with the new
usb-core packet-id generation where qtd addresses are used as ids, causes
duplicate ids for in flight packets.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
(cherry picked from commit 66f092d25697e11847b61d761c38ddebedaed8d1)

Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>