Let iwn(4) simply clear frames before the firmware's BA window, instead

Let iwn(4) simply clear frames before the firmware's BA window, instead
of trying to be smart and clearing already acknowledged frames which are
still within the firmware's BA window.

This matches what the Linux driver does and makes our driver code simpler.

Also, Tx rate control code relies on sequence numbers falling into the
BA window so let's skip Tx rate control for frames before this window.

Tested by:
myself on 6205 and 6300
afresh1, bluhm, and paco on 6300
jmatthew on 5100
Balder Oddson on 6205