iw: Print beacon-loss-event string.

iw: Print beacon-loss-event string.

There are now three possible threshold events.  The old
code assumed only above/below RSSI, and printed out 'low'
for beacon-loss-event.

Fix that, and do so with a switch statement so the compiler
can warn if the enum is further expanded w/out updating this
code.

Shorten 'connection quality monitor' to CQM to keep total string
relatively short.

Signed-off-by: Ben Greear <greearb@candelatech.com>