sfxge(4): improve robustness of MAC stats get via MCDI

Previously the code relied on the callers of efx_mcdi_mac_stats
to provide a DMA buffer or NULL depending on the action. Fix
this so that the DMA buffer is only passed in the request when
needed, and that an error is reported for a missing DMA buffer.

Submitted by: Andy Moreton <amoreton at solarflare.com>
Sponsored by: Solarflare Communications, Inc.
Differential Revision: https://reviews.freebsd.org/D18174