cpufreq: handle cpufreq being disabled for all exported function.

When disable_cpufreq() is called some exported functions are still
being used that do not have a check for cpufreq being disabled.

Add a disabled check into cpufreq_cpu_get() to return NULL if
cpufreq is disabled this covers most of the exported functions. For
the exported functions that do not call cpufreq_cpu_get() add an
explicit check.

Signed-off-by: Dirk Brandewie <dirk.j.brandewie@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>