sfxge(4): use MAC stats DMA buffer size when decoding

On Medford2 and later the MAC stats buffer has been enlarged.
Use the MAC stats DMA buffer size to ensure that the stats END
generation count is read from the correct location, and that
over-reading of the DMA buffer is prevented.

Submitted by: Andy Moreton <amoreton at solarflare.com>
Sponsored by: Solarflare Communications, Inc.
Differential Revision: https://reviews.freebsd.org/D18176