ifnet/DrvAPI: Move if_t typedef to a better place

Summary:
<net/if_var.h> should really be used by the netstack only, not by
drivers. Eventually all the accessors will be moved to <net/if.h> as
well, but for now just move the typedef while the KPI gets sorted and
drivers get converted.

Sponsored by: Juniper Networks, Inc.
Reviewed By: melifaro, glebius
Differential Revision: https://reviews.freebsd.org/D37784