ACPI: processor: Update cpuidle driver check in __acpi_processor_start()

Commit 7a8c994 ("ACPI: processor: idle: Optimize ACPI idle
driver registration") moved the ACPI idle driver registration to
acpi_processor_driver_init() and acpi_processor_power_init() does
not register an idle driver any more.

Accordingly, the cpuidle driver check in __acpi_processor_start() needs
to be updated to avoid calling acpi_processor_power_init() without a
cpuidle driver, in which case the registration of the cpuidle device
in that function would lead to a NULL pointer dereference in
__cpuidle_register_device().

Fixes: 7a8c994 ("ACPI: processor: idle: Optimize ACPI idle driver registration")
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Reviewed-by: Mario Limonciello (AMD) <superm1@kernel.org>
Link: https://patch.msgid.link/5044465.31r3eYUQgx@rafael.j.wysocki
[ rjw: Changelog update ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

Author: rafaeljw

drivers/acpi/processor_driver.c

@@ -166,7 +166,7 @@ static int __acpi_processor_start(struct acpi_device *device)
 	if (result && !IS_ENABLED(CONFIG_ACPI_CPU_FREQ_PSS))
 		dev_dbg(&device->dev, "CPPC data invalid or not present\n");
 
-	if (!cpuidle_get_driver() || cpuidle_get_driver() == &acpi_idle_driver)
+	if (cpuidle_get_driver() == &acpi_idle_driver)
 		acpi_processor_power_init(pr);
 
 	acpi_pss_perf_init(pr);