xhci: remove endpoint ring cache

Anurag Kumar Vulisha reported several issues with xhci endpoint
ring caching.

31 Rings are cached per device before a ring is freed.
These cached rings are not used as default if a new ring is needed.
They are only used if the driver fails to allocate memory for a ring.

The current ring cache is more a reason to why we run out memory than a
help when we actually do so.

Anurag Kumar Vulisha tried to use cached rings as a first option and
found new issues with cached ring initialization.
Cached rings were first zeroed and then manually reinitialized with link
trbs etc, but forgetting to set some important bits like cycle toggle bit.

Remove the ring cache completely as it's a faulty premature optimization
eating memory

Reported-by: Anurag Kumar Vulisha <anuragku@xilinx.com>
Tested-by: Anurag Kumar Vulisha <anuragku@xilinx.com>
Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>