hyperv/hn: Cap default # of rings to 8.

8 gives the best performance in both Azure and local Hyper-V on both
10Ge and 40Ge. More rings are still allowed by manual configuration.

Reviewed by:        Dexuan Cui <decui microsoft com>
MFC after:  1 week
Sponsored by:       Microsoft OSTC
Differential Revision:      https://reviews.freebsd.org/D5879

hyperv/hn: By default enable multiple TX/RX rings, aka vRSS.

Reviewed by:        Dexuan Cui <decui microsoft com>
MFC after:  1 week
Sponsored by:       Microsoft OSTC
Differential Revision:      https://reviews.freebsd.org/D5880

hyperv/hn: Remove unnecessary NULL checks

Submitted by:       Jun Su <junsu microsoft com>
Reviewed by:        sephe
MFC after:  1 week
Sponsored by:       Microsoft OSTC
Differential Revision:      https://reviews.freebsd.org/D5905