usb: xhci: Fix potential memory leak in xhci_disable_slot()

xhci_disable_slot() allows the invoker to pass a command pointer
as paramenter. Otherwise, it will allocate one. This will cause
memory leak when a command structure was allocated inside of this
function while queuing command trb fails. Another problem comes up
when the invoker passed a command pointer, but xhci_disable_slot()
frees it when it detects a dead host.

This patch fixes these two problems by removing the command parameter
from xhci_disable_slot().

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>