hyperv/hn: Fix some ifnet settings

- ifnet.if_mtu does not require explicit setting.
- ifnet.if_hdrlen must be set after ether_ifattach().

Sponsored by:   Microsoft
Differential Revision:  https://reviews.freebsd.org/D7873

hyperv/hn: Regroup ifnet setup code.

While I'm here, add comment along the attach DEVMETHOD.

Sponsored by:   Microsoft
Differential Revision:  https://reviews.freebsd.org/D7874

hyperv/hn: Put debug message under bootverbose

Sponsored by:   Microsoft
Differential Revision:  https://reviews.freebsd.org/D7875