usb: xhci: Return error when host is dead in xhci_disable_slot()

xhci_disable_slot() is a helper for disabling a slot when a device
goes away or recovers from error situations. Currently, it returns
success when it sees a dead host. This is not the right way to go.
It should return error and let the invoker know that disable slot
command was failed due to a dead host.

Fixes: f9e609b8 ("usb: xhci: Add helper function xhci_disable_slot().")
Cc: Guoqing Zhang <guoqing.zhang@intel.com>
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>