sfxge(4): handle negative ticks difference correctly

[arybchik]

Summary

ticks are signed int and if statistics is not updated for a long time
(more than INT_MAX ticks, but less than UINT_MAX) difference becomes
negative and less than hz for a long time.

Other option to repeat is simply load driver (which initializes
timestamps to 0) when ticks are negative.

Sponsored by: Solarflare Communications, Inc.
MFC after: 1 week

Test Plan

Module built, loaded, sysctl stats checked.
Stats tested on a lucky host with negative ticks.