It happened for few frame sizes on 8p gateware with reduced page number,
that SOF was ariving right after EOF (when RCV FSM was in the S_IDLE)
state. This commit adds one register in RCV FSM to store any SOF that
occured before S_IDLE->S_READY transition of RCV_FSM.

The result of this bug was not swcore hang, but missing one SOF, so in
the end we had always one more RTU response than SOFs. Thus every frame
was being forwarded based on the RTU decision for a previous frame..