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>

drivers/cpufreq/cpufreq.c

@@ -2378,7 +2378,7 @@ EXPORT_SYMBOL_GPL(cpufreq_boost_enabled);
  * submitted by the CPU Frequency driver.
  *
  * Registers a CPU Frequency driver to this core code. This code
- * returns zero on success, -EBUSY when another driver got here first
+ * returns zero on success, -EEXIST when another driver got here first
  * (and isn't unregistered in the meantime).
  *
  */