nl80211: fix nl80211: MBSS sharing defaults to on

1. The previous patch didn't do anything.
2. Make NL80211_MESH_SETUP_CAN_SHARE attribute a u8
   (treated as boolean), so the intent becomes more clear
   (since "shared" defaults to true, we'd need a flag just
   to turn it off, which isn't that intuitive).

Signed-off-by: Thomas Pedersen <thomas@cozybit.com>