MFC: r343203

ixgbe: this statement may fall through warnings with gcc

The recent gcc versions (7 and 8 at least) can check for switch case
statements for fall through (implicit-fallthrough). When fall through
is intentional, the default method for warning suppression is to place
comment /* FALLTHROUGH */ exactly before next case statement.

Differential Revision: https://reviews.freebsd.org/D18577