hwmon: (nct7802) fix visibility of temp3

Excerpt from datasheet:
7.2.32 Mode Selection Register
RTD3_MD : 00=Closed , 01=Reserved , 10=Thermistor mode , 11=Voltage sense

Show temp3 only in Thermistor mode

Cc: stable@vger.kernel.org # v3.19+
Signed-off-by: Constantine Shulyupin <const@MakeLinux.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>