[SERIAL] ACPI serial fix

Patch from Bjorn Helgaas

Intel 870 firmware reports an extra zero-length IO port range, which
is bogus, as far as I can tell.  Ignore it.

drivers/serial/8250_acpi.c

@@ -38,8 +38,11 @@ static acpi_status acpi_serial_mmio(struct serial_struct *req,
 static acpi_status acpi_serial_port(struct serial_struct *req,
 				    struct acpi_resource_io *io)
 {
+	if (io->range_length) {
 		req->port = io->min_base_address;
 		req->io_type = SERIAL_IO_PORT;
+	} else
+		printk(KERN_ERR "%s: zero-length IO port range?\n", __FUNCTION__);
 	return AE_OK;
 }