mac80211: make beacon-loss-count configurable

On loaded systems with lots of VIFs, I see lots of beacon
timeouts, even though the connection to the AP is very
good.  Allow tuning the beacon-loss-count variable to
give the system longer to process beacons if the user
prefers.

Signed-off-by: Ben Greear <greearb@candelatech.com>
[add the number of beacons to the message]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>