ACPI: EC: Make acpi_ec_ecdt_probe() more straightforward

Since acpi_ec_ecdt_probe() is called when boot_ec is not set, it
doesn't neeed to take the other possibility into account.

Accordingly, it only needs to set the handle field in the ec object
to ACPI_ROOT_OBJECT, call acpi_ec_setup() and (if that is successful)
set boot_ec to ec and boot_ec_is_ecdt to 'true'.  Make it do so
directly, without calling acpi_config_boot_ec(), and avoid some
pointless checks in the latter.

No intentional functional impact except for a changed message.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>