et131x: use msecs_to_jiffies for conversions

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

Signed-off-by: Nicholas Mc Guire <hofrat@osadl.org>
Acked-by: Mark Einon <mark.einon@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>