MFC r340802

sfxge(4): use proper MCDI command for encap filters

MC_CMD_FILTER_OP_IN_EXT is needed to set filters for encapsulated
packets.

Submitted by: Mark Spender <mspender at solarflare.com>
Sponsored by: Solarflare Communications, Inc.
Differential Revision: https://reviews.freebsd.org/D18072