MFC r294253

sfxge: if supported by firmware, use enhanced SET_MAC command to only configure the MTU

This allows an MTU change to be requested on unpriviliged functions
without also setting all the other parameters supported by MC_CMD_SET_MAC.

The enhanced SET_MAC command was introduced in v4_7 firmware.

Submitted by: Mark Spender <mspender at solarflare.com>
Sponsored by: Solarflare Communications, Inc.