cfg80211: Advertise maximum associated STAs in AP mode

This allows drivers to advertise the maximum number of associated
stations they support in AP mode (including P2P GO). User space
applications can use this for cleaner way of handling the limit (e.g.,
hostapd rejecting IEEE 802.11 authentication without manual
configuration of the limit) or to figure out what type of use cases can
be executed with multiple devices before trying and failing.

Signed-off-by: Jouni Malinen <j@w1.fi>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>