mac80211: shorten the IBSS debug messages

When tracing is enabled, all the debug messages are recorded and must
not exceed MAX_MSG_LEN (100) columns. Longer debug messages grant the
user with:

WARNING: CPU: 3 PID: 32642 at /tmp/wifi-core-20180806094828/src/iwlwifi-stack-dev/net/mac80211/./trace_msg.h:32 trace_event_raw_event_mac80211_msg_event+0xab/0xc0 [mac80211]
Workqueue: phy1 ieee80211_iface_work [mac80211]
 RIP: 0010:trace_event_raw_event_mac80211_msg_event+0xab/0xc0 [mac80211]
 Call Trace:
  __sdata_dbg+0xbd/0x120 [mac80211]
  ieee80211_ibss_rx_queued_mgmt+0x15f/0x510 [mac80211]
  ieee80211_iface_work+0x21d/0x320 [mac80211]

Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>