hwmon: (coretemp) Fixup platform device ID change

With recent change "hwmon: (coretemp) don't use kernel assigned CPU
number as platform device ID", the microcode check is now running on
random CPU. Fix that by checking the microcode before creating the
platform device rather than at probe time.

Also avoid calling TO_PHYS_ID(cpu) twice in the same function, it's
expensive.

Signed-off-by: Jean Delvare <khali@linux-fr.org>
Cc: Jan Beulich <jbeulich@suse.com>
Cc: Guenter Roeck <guenter.roeck@ericsson.com>
Signed-off-by: Guenter Roeck <guenter.roeck@ericsson.com>