platform/chrome: cros_ec: honor acpi=off

when acpi=off is provided in bootarg, kernel crash with

 BUG: kernel NULL pointer dereference, address: 0000000000000018
 RIP: 0010:acpi_ns_walk_namespace+0x57/0x280
  <TASK>
 ? acpi_get_devices+0x140/0x140
 cros_ec_lpc_init+0x25/0x100

Driver should check if ACPI is disabled before calling acpi_get_devices(),
otherwise acpi_walk_namespace() will dereference null pointer since the
acpi_gbl_root_node is not initialized.
This is a common pattern and should be fixed in ACPI framework to prevent
such crash in the future, but since many drivers are already doing explicit
check(acpi_disable) we do the same thing here.

Signed-off-by: Tong Zhang <ztong0001@gmail.com>

drivers/platform/chrome/cros_ec_lpc.c

@@ -550,6 +550,9 @@ static int __init cros_ec_lpc_init(void)
 	int ret;
 	acpi_status status;
 
+	if (acpi_disable)
+		return -ENODEV;
+
 	status = acpi_get_devices(ACPI_DRV_NAME, cros_ec_lpc_parse_device,
 				  &cros_ec_lpc_acpi_device_found, NULL);
 	if (ACPI_FAILURE(status))