sfxge(4): add X2 port modes to bandwidth calculator

Add cases for the new port modes supported by X2 NICs.
Lane bandwidth is calculated for pre-X2 cards so is an
underestimate for X2 in 25G/100G modes.

Submitted by: Richard Houldsworth <rhouldsworth at solarflare.com>
Sponsored by: Solarflare Communications, Inc.
Differential Revision: https://reviews.freebsd.org/D18277