hwmon: (nct7802) Fix wrong detection of in4 presence

The code to detect if in4 is present is wrong; if in4 is not present,
the in4_input sysfs attribute is still present.

In detail:

- Ihen RTD3_MD=11 (VSEN3 present), everything is as expected (no bug).
- If we have RTD3_MD!=11 (no VSEN3), we unexpectedly have a in4_input
  file under /sys and the "sensors" command displays in4_input.
  But as expected, we have no in4_min, in4_max, in4_alarm, in4_beep.

Fix is_visible function to detect and report in4_input visibility
as expected.

Reported-by: Gilles Buloz <Gilles.Buloz@kontron.com>
Cc: Gilles Buloz <Gilles.Buloz@kontron.com>
Cc: stable@vger.kernel.org
Fixes: 3434f378

 ("hwmon: Driver for Nuvoton NCT7802Y")
Signed-off-by: Guenter Roeck <linux@roeck-us.net>