dpaa2: Setup interface caps on attach

[dsl]

Summary

I've recently discovered (to my surprise) that my jail on Honeycomb LX2 sends UDP and TCP packets with incorrect checksums over epair(4). Bisecting led me to 39d4094173f9 ("epair: add support for checksum offloading"). There's a "dpaa2_ni_setup_if_caps" function in dpaa2_ni.c which configures L3/L4 checksums HW offloading, but it wasn't called in my case during the boot process at all. However, toggling capabilities on the working dpni0 interface helped. I call "dpaa2_ni_setup_if_caps" as a part of the dpaa2_ni attach here.

Test Plan

1. Run the latest CURRENT (e.g. 6fa18fe74461);
2. $ drill freebsd.org;
3. Observe proper address resolved;
4. $ jexec <JAIL_ID> /bin/sh;
5. $ drill freebsd.org;
6. Observe a delay and a network error.