[CPUFREQ] Bugfix: Call driver exit in cpufreq_add_dev error path

A minor fix for cpufreq_add_dev() error path. We need to call driver->exit()
if driver_init() call has succeeded.

Signed-off-by: Venkatesh Pallipadi <venkatesh.pallipadi@intel.com>
Signed-off-by: Dave Jones <davej@redhat.com>