hyperv/hn: Staticize and rename packet TX done function

It is only used in hv_netvsc_drv_freebsd.c; and rename it to hn_tx_done()
mainly to reserve "xmit" for ifnet.if_transmit implement.

While I'm here, remove unapplied comment.

Reviewed by: adrian
Approved by: adrian (mentor)
MFC after: 1 week
Sponsored by: Microsoft OSTC
Differential Revision: https://reviews.freebsd.org/D5345