hwmon: (ntc_thermistor) Improve precision of resistance calculation

The function get_ohm_of_thermistor has both the measured voltage and the
pullup voltage available in microvolts.  But it was promptly converting
both to millivolts before using them to calculate the thermistor
resistance.  That conversion unnecessarily hurt the precision of the
calculation.

For example, take the ncpXXwb473 connected to 5000 mV and pulled down
through a 47000 ohm resistor.  At 25 C, the resistance of the thermistor
is 47000 ohms.  The measured voltage will be 2500 mV.  If we measure
instead 2501 mV, then the calculated resistance will be 46962 ohms --
a difference of 38 ohms.  So the precision of the resistance estimate
could be increased by 38X by doing the calculations in microvolts.

Signed-off-by: Chris Lesiak <chris.lesiak@licor.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>