mesh_plink: use msecs_to_jiffies for proper time conversion

This is primarily an API consolidation and should make things more readable
it replaces var * HZ / 1000 by msecs_to_jiffies(var) which also handles
corner cases correctly.

There is a change of behavior as e.g. for HZ 100, t * HZ / 1000 will
return 0 for t < 10 but msecs_to_jiffies will return at least 1 always.

Signed-off-by: Nicholas Mc Guire <hofrat@osadl.org>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>