on receiving some frame sizes we lose a few words from minic frame buffer

Turns out that MiniNIC receiving FSM was sometimes going through RX_MEM_FLUSH when it was not needed, thus writing the last data word twice to the frame buffer. However, the frame size was not being incremented, so software after receiving frame was not freeing less than actually used in the buffer.
This way, after some time minic was dropping frames because it though its buffer didn't have enough space (based on nrx_avail value).

This bug is fixed with commit 221a6143.