powerpc/therm_adt746x: Don't access non-existing register

The ADT746x don't have any register at sub-address 0, so better use an
existing register for the initial test read.

Signed-off-by: Jean Delvare <khali@linux-fr.org>
Cc: Colin Leroy <colin@colino.net>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Paul Mackerras <paulus@samba.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>