cpufreq: acpi-cpufreq: Skip initialization if cpufreq driver is present

cpufreq: acpi-cpufreq: Skip initialization if cpufreq driver is present

Revert part of commit 75c0758137c7a ("acpi-cpufreq: Fail initialization
if driver cannot be registered").

acpi-cpufreq is mutually exclusive with intel_pstate, but
acpi-cpufreq is loaded multiple times during startup while
intel_pstate is enabled.

On systems using systemd the kernel triggers one uevent for each
device as a result of systemd-udev-trigger.service. The service
exists to retrigger all devices as uevents sent by the kernel before
systemd-udevd is running are missed.

The delay caused by systemd-udevd repeatedly loading the driver,
getting a fail return, and unloading the driver twice per logical
CPU has a significant impact on the startup time, and can cause some
devices to be unavailable after reaching the root login prompt.

Load the driver once but skip initialization if a cpufreq driver
exists by changing the return value of cpufreq_get_current_driver()
from -EEXIST to 0.

Fixes: 75c0758137c7a ("acpi-cpufreq: Fail initialization if driver cannot be registered")
Signed-off-by: Kyle Meyer <kyle.meyer@hpe.com>
[ rjw: Subject and changelog edits ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>