usb: xhci: use list_is_singular for cmd_list

Use list_is_singular() to check if cmd_list has only one entry.

[use list_empty() in queue command instead -Mathias]
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>