fix datapath Rx buffer management in qwx(4)

fix datapath Rx buffer management in qwx(4)

Fixes Tx/Rx stalls where the device ran out of free Rx buffers to use.

The device consumes buffers on the Rx refill ring out of order, which
the ring management code I wrote intially would not handle properly.
Instead of using an index into the ring where we would expect to see
a free slot which was in fact occupied, keep track of free buffers via
a bitmap.