net: dsa: use DEVICE_ATTR_RW to declare temp1_max

Since commit da4759c7

 (sysfs: Use only return value from is_visible for
the file mode), it is possible to reduce the permissions of a file.

So declare temp1_max with the DEVICE_ATTR_RW macro and remove the write
permission in dsa_hwmon_attrs_visible if set_temp_limit isn't provided.

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: David S. Miller <davem@davemloft.net>