cfg80211: allow adding/deleting stations on mesh

Commit b2a151a288 added a check that prevents adding or deleting
stations on non-AP interfaces.  Adding and deleting stations is
supported for Mesh Point interfaces, so add Mesh Point to that check as
well.

Signed-off-by: Andrey Yurovsky <andrey@cozybit.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>