sfxge(4): retrieve number of MAC stats from NIC

This reports the number of stats (and hence the DMA buffer size)
for MAC stats. If MC_GET_CAPABABILITIES_V4 is not supported then
use the legacy Siena-compatible MC_CMD_MAC_NSTATS value.

Submitted by: Andy Moreton <amoreton at solarflare.com>
Sponsored by: Solarflare Communications, Inc.
Differential Revision: https://reviews.freebsd.org/D18173