hwmon: (lm95245) Make temp2_crit_hyst read-only

The hysteresis register is shared among both temperature sensors.
This means changing one also affects the other. To avoid confusion,
established way to express this is to make only the first instance writable
and keep all other instances as read-only. Otherwise users may be
confused that changing the second writable value also affects the first,
while it is more obvious that a writable value may affect a different
read-only value.

Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Reviewed-by: Jean Delvare <jdelvare@suse.de>