cpufreq: fix comment about return value of cpufreq_register_driver()

The comment has been incorrect since commit 4dea5806
("cpufreq: return EEXIST instead of EBUSY for second registering").

Signed-off-by: Eric Biggers <ebiggers3@gmail.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>