hwmon: (lm90) Use programmed update rate

The lm90 driver programs the sensor chip to update its readings at 2 Hz
(500 ms between readings). However, the driver only does reads from the
chip at intervals of 2 * HZ (2000 ms between readings). Change the driver
update rate to the programmed update rate.

Signed-off-by: Ira W. Snyder <iws@ovro.caltech.edu>
Signed-off-by: Jean Delvare <khali@linux-fr.org>