ab8500_fg: use jiffies_to_msecs for jiffies conversion

Converting jiffies to milliseconds by "val * 1000 / HZ" is technically
OK but jiffies_to_msecs(val) is the cleaner solution and handles all
corner cases correctly. This is a minor API consolidation only and
should make things more readable.

Signed-off-by: Nicholas Mc Guire <hofrat@osadl.org>
Signed-off-by: Sebastian Reichel <sre@kernel.org>