usb: xhci: Fix memory leak when xhci_disable_slot() returns error

If xhci_disable_slot() returns success, a disable slot command
trb was queued in the command ring. The command completion
handler will free the virtual device data structure associated
with the slot. On the other hand, when xhci_disable_slot()
returns error, the invokers should take the responsibilities to
free the slot related data structure. Otherwise, memory leakage
happens.

Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>