ACPI: remove superfluous NULL pointer check from acpi_processor_get_throttling_info()

Dan's list contains:

drivers/acpi/processor_throttling.c +1139 acpi_processor_get_throttling_info(11) warning: variable derefenced before check 'pr'

acpi_processor_get_throttling_info() is never called with pr == NULL.

[ bart: the potential NULL pointer dereference was finally fixed in
  (much later than mine) commit 5cfa245b but my patch is still valid ]

Reported-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Len Brown <len.brown@intel.com>