devfreq_class is used internally by devfreq and has no
need to be globally available.

This also fixes the following sparse warning:
drivers/devfreq/devfreq.c:30:14: warning: symbol 'devfreq_class' was not declared. Should it be static?
Signed-off-by: NNishanth Menon <nm@ti.com>
Signed-off-by: NRafael J. Wysocki <rafael.j.wysocki@intel.com>