mac80211: Remove mesh paths when an interface is removed

When an interface is removed, the mesh paths associated with it should
also be removed.

This fixes a bug we observed when reloading a device driver module
without reloading mac80211s.

Signed-off-by: Javier Cardona <javier@cozybit.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>