xhci: drop ER_FULL_HACK workaround

The nec/renesas driver problems have finally been debugged and root
caused, see commit "7da76e12 xhci: fix event queue IRQ handling".

It's pretty clear now that
 (a) The whole "driver can't handle ring full" story is most likely
     wrong.
 (b) The ER_FULL_HACK workaround based on the false assumtion doesn't
     much.  It avoids the driver crashing (without commit 7da76e12), but
     it doesn't make usb work.
 (c) With 7da76e12 applied it doesn't trigger any more.

So, lets kill it.  Or, to be exact, lets almost kill it.  Some data
fields are kept unused in the state struct, for live migration backward
compatibility.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Message-id: 1486382139-30630-2-git-send-email-kraxel@redhat.com