[MAC80211]: Check open_count before calling config callback.

Also remove the check for ops->config!=NULL, as it can never be NULL.

Signed-off-by: Michael Buesch <mb@bu3sch.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>