cpufreq: return EEXIST instead of EBUSY for second registering

On systems that support intel_pstate, acpi_cpufreq fails to load, and
udev keeps trying until trace gets filled up and kernel crashes.

The root cause is driver return ret from cpufreq_register_driver(),
because when some other driver takes over before, it will return
EBUSY and then udev will keep trying ...

cpufreq_register_driver() should return EEXIST instead so that the
system can boot without appending intel_pstate=disable and still use
intel_pstate.

Signed-off-by: Yinghai Lu <yinghai@kernel.org>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>