MFC r291391

sfxge: do not use unnamed union in siena_mc_combo_rom_hdr_t

GCC 4.2.1 used on FreeBSD 8 and 9 branches does not like unnamed
union member in the structure. It is not strictly required in head,
but nice to have to minimize difference with out-of-tree driver.

Sponsored by: Solarflare Communications, Inc.