wl12xx: support FW TX inactivity triggers

In AP mode we register for the MAX_TX_RETRY and INACTIVE_STA events.
Both are reported to the upper layers as a TX failure in the offending
stations.

In STA mode we register only for the MAX_TX_RETRY event. A TX failure is
interpreted as a loss of connection.

Support for IEEE80211_HW_REPORTS_TX_ACK_STATUS has been removed to avoid
the inherent race condition of a mac80211 TX failure counter in addition
to the FW counter.

This patch depends on "mac80211: allow low level driver to report packet
loss"

Signed-off-by: Arik Nemtsov <arik@wizery.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>