drivers/cpufreq/pcc-cpufreq.c: avoid NULL pointer dereference

drivers/cpufreq/pcc-cpufreq.c: avoid NULL pointer dereference

per_cpu(processors, n) can be NULL, resulting in:

  Loading CPUFreq modules[  437.661360] BUG: unable to handle kernel NULL pointer dereference at (null)
  IP: [<ffffffffa0434314>] pcc_cpufreq_cpu_init+0x74/0x220 [pcc_cpufreq]

It's better to avoid the oops by failing the driver, and allowing the
system to boot.

Signed-off-by: Naga Chumbalkar <nagananda.chumbalkar@hp.com>
Cc: Dave Jones <davej@codemonkey.org.uk>
Cc: Len Brown <lenb@kernel.org>
Cc: <stable@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>