dpaa2: allow tapping of tx packets in dpni

Packet capturing on dpni is only half-working given the BPF_MTAP call
in the TX path is missing. Add it to see packets in both directions.

Reviewed by: dsl
Differential Revision: https://reviews.freebsd.org/D47103

(cherry picked from commit e69e172d40585cfbcfdb28433cec523ca4867cf1)