Reset protocol level RX frame delta after reading
Handle cases where the protocol stops processing valid frames. Before this fix the last valid frame interval would continue to be reported if no more valid frames were received. No calculation will fallback to the interval measured in the RX task which will ultimately get set to the max task interval (when no data is received) or 30ms.
Change to capturing the last valid frame time rather than the delta in the protocol level. Move the common calculation of the delta up into rx.c.