ACPI: EC: Fix up fast path check in acpi_ec_add()

The fast path check in acpi_ec_add() is not incorrect, because in
fact acpi_device_hid(device) can be equal to ACPI_ECDT_HID only if
boot_ec is not NULL, but it may confuse static checkers, so change
it to explicitly check boot_ec upfront and use the slow path if
that pointer is NULL.

Link: https://lore.kernel.org/linux-acpi/20200406144217.GA68494@mwanda/
Fixes: 3d9b8dd8 ("ACPI: EC: Use fast path in acpi_ec_add() for DSDT boot EC")
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

drivers/acpi/ec.c

@@ -1589,8 +1589,8 @@ static int acpi_ec_add(struct acpi_device *device)
 	strcpy(acpi_device_name(device), ACPI_EC_DEVICE_NAME);
 	strcpy(acpi_device_class(device), ACPI_EC_CLASS);
 
-	if ((boot_ec && boot_ec->handle == device->handle) ||
-	    !strcmp(acpi_device_hid(device), ACPI_ECDT_HID)) {
+	if (boot_ec && (boot_ec->handle == device->handle ||
+	    !strcmp(acpi_device_hid(device), ACPI_ECDT_HID))) {
 		/* Fast path: this device corresponds to the boot EC. */
 		ec = boot_ec;
 	} else {