[MAC80211]: only honor IW_SCAN_THIS_ESSID in STA, IBSS, and AP modes

The previous IW_SCAN_THIS_ESSID patch left a hole allowing scan
requests on interfaces in inappropriate modes.

Signed-off-by: John W. Linville <linville@tuxdriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>