hyperv/hn: Factor out func to exec RNDIS transaction w/o checking result

It will be used by RNDIS HALT and RESET.

Sponsored by:   Microsoft
Differential Revision:  https://reviews.freebsd.org/D7685

hyperv/hn: Switch to new RNDIS transaction execution for halt.

Sponsored by:   Microsoft
Differential Revision:  https://reviews.freebsd.org/D7686