hwmon: (pmbus) Fix temperature limit register access

Commit 8677011a added auto-update to temperature limit registers.
Unfortunately, the update flag is also used to determine if an attribute
is writable, which results in read-only temperature limit registers.
To fix the problem, pass 'readonly' as separate flag to the function used
to add sensor attributes.
Signed-off-by: Guenter Roeck <guenter.roeck@ericsson.com>