sfxge(4): add helper to compute timer quantum

[arybchik]

Summary

This also adjusts the timer values used to match the Linux net
driver implementation:
a) non-zero time intervals should result in at least one quantum
b) timer load/reload values are only zero biased for Falcon/Siena

Submitted by: Andy Moreton <amoreton at solarflare.com>
Sponsored by: Solarflare Communications, Inc.
MFC after: 1 week

Test Plan

Module built, loaded, interface up, IPv4 traffic work.
amd64 tinderbox kernels is in progress