hwmon/w83627ehf: Preserve speed reading when changing fan min

The w83627ehf driver changes the fan clock divider automatically when
a new min fan speed is set. It is supposed to preserve the fan speed
reading while doing so, bug doesn't really. Fix it.

Signed-off-by: Jean Delvare <khali@linux-fr.org>
Signed-off-by: Mark M. Hoffman <mhoffman@lightlink.com>