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

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: e9e1a9080852 ("ACPI: EC: Use fast path in acpi_ec_add() for DSDT boot EC")
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>