mac80211: fix crash if bitrate calculation goes wrong

If a frame's timestamp is calculated, and the bitrate
calculation goes wrong and returns zero, the system
will attempt to divide by zero and crash. Catch this
case and print the rate information that the driver
reported when this happens.

Cc: stable@vger.kernel.org
Reported-by: Thomas Lindroth <thomas.lindroth@gmail.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>