mac80211: Rewrote code for checking if destinations are proxied.

Rewrote code for checking if the destination is proxied by a mesh portal, to facilitate better
understanding of the functionality.

Signed-off-by: Joel A Fernandes <agnel.joel@gmail.com>
Acked-by: Javier Cardona <javier@cozybit.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>