hwmon: (gl518sm) Fix the reported fan speed

The fan speeds reported by the gl518sm driver are twice as much as they
should. It's currently reporting the number of pulses per minute, not
rotations per minute, while typical fans emit two pulses per rotation.
This explains why all reports with this driver had very high speed
values (between 9000 to 12000 RPM). Odd that nobody ever actually
complained about this bug.

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