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

[arybchik]

Summary

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.

Test Plan

amd64 tinderbox kernel builds tested
driver loaded/unloaded, interfaces up/down, traffic works fine