hyperv/hn: Fixat RNDIS rxfilter after the successful RNDIS init.

Under certain conditions on certain versions of Hyper-V, the RNDIS
rxfilter is _not_ zero on the hypervisor side after the successful
RNDIS initialization, which breaks the assumption of any following
code (well, it breaks the RNDIS API contract actually). Clear the
RNDIS rxfilter explicitly, drain packets sneaking through, and drain
the interrupt taskqueues scheduled due to the stealth packets.

Reported by: dexuan@
MFC after: 3 days
Sponsored by: Microsoft
Differential Revision: https://reviews.freebsd.org/D10230