IfAPI: Style cleanup

Summary:
Clean up style issues from IfAPI additions.

Casts to (struct ifnet *) made sense when if_t was a void *, but
since it's a struct ifnet * it no longer makes sense. Fix whitespace
errors, among others.

Reviewed by: kib, glebius
Sponsored by: Juniper Networks, Inc.
Differential Revision: https://reviews.freebsd.org/D38499