mac80211: Don't use a buf_size=0 in ADDBA requests

According to 802.11-2007, 7.3.1.14 it is compliant to use a buf_size of
0 in ADDBA requests. But some devices (AVM Fritz Stick N) arn't able to
handle that correctly and will reply with an ADDBA reponse with a
buf_size of 0 which in turn will disallow BA sessions for these
devices.

To work around this problem, initialize hw.max_tx_aggregation_subframes
to the maximum AMPDU buffer size 0x40.

Using 0 as default for the bufsize was introduced in commit
5dd36bc933e8be84f8369ac64505a2938f9ce036 (mac80211: allow advertising
correct maximum aggregate size).

Signed-off-by: Helmut Schaa <[e-mail]>
Acked-by: Johannes Berg <[e-mail]>
Signed-off-by: John W. Linville <[e-mail]>

net/mac80211/main.c

···
 	local->hw.max_rates = 1;
 	local->hw.max_report_rates = 0;
 	local->hw.max_rx_aggregation_subframes = IEEE80211_MAX_AMPDU_BUF;
+	local->hw.max_tx_aggregation_subframes = IEEE80211_MAX_AMPDU_BUF;
 	local->hw.conf.long_frame_max_tx_count = wiphy->retry_long;
 	local->hw.conf.short_frame_max_tx_count = wiphy->retry_short;
 	local->user_power_level = -1;