If the Disable Slot command fails then the hardware may continue to
write to the slot context.  Leak the memory used by the slot context
to avoid future memory corruption.

This situation has been observed in practice when a Set Address
command fails, causing the command ring to become temporarily
unresponsive.

Note that there is no need to similarly leak memory on the failure
path in xhci_device_open(), since in the event of a failure the
hardware is never informed of the slot context address.

Signed-off-by: Michael Brown <mcb30@ipxe.org>