nimble/ll: Fix sending HCI Number of Completed Packets event

[andrzej-kaczmarek]

The calculations for sending this HCI event periodically seem strange,
probably something was mixed up in the past here.

After fix I believe this does what it is supposed to do: every X ticks
we'll just go through all connections and send HCI event no matter if
there is something to send or not.

Also added some with reference to spec section.

[wes3]

Does the current code not work? Seems like it should do what it was intended to do: only poll the list of all connections no more often than the completed packet rate but send an event for the current connection if any packets were completed.

[andrzej-kaczmarek]

Ok, so I had to write this step-by-step on a paper in order to find out that it seems to work, but I find it really hard to understand - especially the part that 'rate' is added to 'last time' at the end of function and the fact that BLE_NUM_COMP_PKT_RATE is defined in ticks but in code it is multiplied by OS_TICKS_PER_SEC only add to the mystery :-)

Also I found an issue with my change as well so in new patch version I updated only two things I find most difficult to follow.

[wes3]

I certainly agree the original code is very confusing and could be written differently to make it clearer. I would have used a timer for it but wanted to save memory. So all good...