Update receive_check.ino

Checking CAN.checkReceive() agains MC_STAT_RXIF_MASK doesn't seem to make sense, it was confusing to me because the code seemed to be using equality with a mask. Since checkReceive() actually returns CAN_MSGAVAIL or CAN_NOMSG, Comparison with CAN_MSGAVAIL seemed more informative than MC_STAT_RXIF_MASK.