hwmon: (it87) Improve temperature reporting support

Add test if thermistor sensor type attribute should be visible, i.e.
test if the attribute is defined.

Signed-off-by: Frank Crawford <frank@crawford.emu.id.au>
Link: https://lore.kernel.org/r/20230707123005.956415-3-frank@crawford.emu.id.au


[groeck: Dropped unnecessary 'type' variable in it87_temp_is_visible()]
Signed-off-by: Guenter Roeck <linux@roeck-us.net>