sfxge(4): support FW subvariant choice

If DPDK application or OS does not need checksumming on transmit,
it may be disabled in firmware to achieve higher packet rates.
Choice must be done before VIS allocation and is allowed if
no other non-preboot and firmware subvariant-unaware drivers are
attached.

Sponsored by: Solarflare Communications, Inc.
Differential Revision: https://reviews.freebsd.org/D18227