mac802154: remove unneeded vif struct

This patch removes the virtual interface structure from sub if data
struct, because it isn't used anywhere. This structure could be useful
for give per interface information at softmac driver layer. Nevertheless
there exist no use case currently and it contains the interface type
information currently. This information is also stored inside wpan dev
which is now used to check on the wpan dev interface type.

Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Reviewed-by: Varka Bhadram <varkabhadram@gmail.com>
Acked-by: Varka Bhadram <varkabhadram@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>