Initialise the pps / packet tracking timestamp so 11n aggregation works again.

There's a bug in the ticks handling where when initialised at '0', once
the ticks counter wrapped the comparison math would never trigger.
The pps calculation would never happen, and thus aggregation was never
enabled.

It manifests itself as "oh you only get 11n transmit aggregation for the
first 10 minutes of uptime."

I'm sure there are other ticks related issues lurking in net80211.

Tested:

• ath / iwn, both with 'wlandebug +11n' and a little bit of iperf to kick off the transmit A-MPDU negotiation once the pps gets high enough.