BugLink: http://bugs.launchpad.net/bugs/1745266

This is a stable-only fix for the backport of commit 5d9b70f7
("xhci: Don't add a virt_dev to the devs array before it's fully
allocated").

In branches that predate commit c5628a2a ("xhci: remove endpoint
ring cache") there is an additional failure path in
xhci_alloc_virt_device() where ring cache allocation fails, in
which case we need to free the ring allocated for endpoint 0.
Signed-off-by: Ben Hutchings <ben.hutchings@codethink.co.uk>
Cc: Mathias Nyman <mathias.nyman@intel.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>