ACPI / processor_thermal: avoid null pointer deference error

Fix a null pointer deference by acpi_driver_data() if device is
null.  We should only set pr and check this is OK after we are
sure device is not null.

Smatch analysis:

drivers/acpi/processor_thermal.c:223 processor_get_max_state() warn:
  variable dereferenced before check 'device' (see line 221)
drivers/acpi/processor_thermal.c:237 processor_get_cur_state() warn:
  variable dereferenced before check 'device' (see line 235)
drivers/acpi/processor_thermal.c:255 processor_set_cur_state() warn:
  variable dereferenced before check 'device' (see line 251)

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>