hyperv/hn: Fix ifnet hwassist setup.

Sponsored by:   Microsoft
Differential Revision:  https://reviews.freebsd.org/D7948

hyperv/hn: Let the caller of hn_nvs_doinit() do the error logging.

So that NVS version probing failure does not look too scary.

Sponsored by:   Microsoft
Differential Revision:  https://reviews.freebsd.org/D7950