hyperv/hn: Stringent RSS sysctl checks

• Don't change RNDIS RSS configuration for RSS key sysctl, if the interface is not capable of RSS yet.
• Don't change RSS indirect table (both cached one and RNDIS RSS configuration), if the interface is not capable of RSS yet.

MFC after: 1 week
Sponsored by: Microsoft
Differential Revision: https://reviews.freebsd.org/D7924