Field is generated in the FRAME. On transmission field is not
inserted (of course, the Protocol control decodes it by itself).
Sanity test stores RWCNT to TX memories and reads it back. Thus
on TX RWCNT is in TX memorz from frame generation. on RX it is
in RX Memory since it was read from the controller. So it is a
simple check that HW returns the same RWCNT as SW generated frame has...