sfxge(4): fix MAC Tx stats for less or equal to 64 bytes

This statistic should include 64byte and smaller frames.
Fix EF10 calculation to match Siena code.

Submitted by: Andy Moreton <amoreton at solarflare.com>
Sponsored by: Solarflare Communications, Inc.
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D18271