hwmon/w83627ehf: Fix timing issues

* I have experimental evidence that the W83627EHG needs more than 1
second to refresh all the measured values. Increase the caching time to
1.5 second.
* When changing a fan clock divider, the corresponding fan speed
measurement register is no longer valid, until the next time the chip
will refresh it. One way to fix this is to pretend that the cache is
still valid for one more period (1.5 second.)

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