hyperv/hn: remember the channel pointer in struct hn_rx_ring

This will be used by the coming NIC SR-IOV patch.

Reviewed by:	sephe
Approved by:	sephe (mentor)
Sponsored by:	Microsoft
Differential Revision:	https://reviews.freebsd.org/D8909

hyperv/hn: remove the MTU and IFF_DRV_RUNNING checking in hn_rxpkt()

It's unnecessary because the upper nework stack does the same checking.

In the case of Hyper-V SR-IOV, we need to remove the checking because
1) multicast/broadcast packets are still received through the synthetic
NIC and we need to inject the packets through the VF interface;
2) we must inject the packets even if the synthetic NIC is down, or has
a different MTU from the VF device.

Reviewed by:	sephe
Approved by:	sephe (mentor)
Sponsored by:	Microsoft
Differential Revision:	https://reviews.freebsd.org/D8962