cpufreq: integrator: fix integrator_cpufreq_remove return type

When building this driver as a module, we get a helpful warning
about the return type:

drivers/cpufreq/integrator-cpufreq.c:232:2: warning: initialization from incompatible pointer type
  .remove = __exit_p(integrator_cpufreq_remove),

If the remove callback returns void, the caller gets an undefined
value as it expects an integer to be returned. This fixes the
problem by passing down the value from cpufreq_unregister_driver.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: All applicable <stable@vger.kernel.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>