Correct off-by-one error in encoder reading

ball_and_beam-2018/avr/ball_and_beam-2018.c

@@ -142,7 +142,7 @@ SIGNAL(USART_RXC_vect)
       }
     } break;
     case serialio_pollchannel: { 
-      if (serialio_channel < 6) { 
+      if (serialio_channel < 7) {
         serial_readchannels |= (1<<serialio_channel); 
       } else if (serialio_channel == 31) { 
         serial_readconfig = 1;