e1000g: this statement may fall through

The gcc 7 does check for switch statement fall through cases, and if legit,
such complaint can besilenced by /* FALLTHROUGH */ comment. Unfortunately
such comment is quite limited, but will still notify the reader.

This patch is backport from illumos, see
https://www.illumos.org/rb/r/941/

Reviewed by: eadler
Differential Revision: https://reviews.freebsd.org/D14663