net/rndis: Add comment for rndis_comp_hdr

Reviewed by:    hps
Sponsored by:   Microsoft
Differential Revision:  https://reviews.freebsd.org/D7738

net/rndis: Define common message header for RNDIS messages.

And avoid RNDIS_HEADER_OFFSET hardcoding.

Reviewed by:    hps
Sponsored by:   Microsoft
Differential Revision:  https://reviews.freebsd.org/D7739