Improve the communication with Darwin Air devices.

We received a report of a Darwin Air device which has a very high error
rate. The majority of the echo packets is incorrect, but since this
doesn't seem to have any effect on the actual data packet, we can just
ignore this error. If there happens to be a more serious error, it will
be detect in the data packet.

Sometimes there were also a some garbage bytes received at startup.
Adding a small delay seems to fix this.

src/mares_common.c

@@ -152,7 +152,6 @@ mares_common_packet (mares_common_device_t *device, const unsigned char command[
 		// Verify the echo.
 		if (memcmp (echo, command, csize) != 0) {
 			WARNING ("Unexpected echo.");
-			return DEVICE_STATUS_PROTOCOL;
 		}
 	}
 
@@ -196,6 +195,9 @@ mares_common_transfer (mares_common_device_t *device, const unsigned char comman
 		// Abort if the maximum number of retries is reached.
 		if (nretries++ >= MAXRETRIES)
 			return rc;
+
+		// Discard any garbage bytes.
+		serial_flush (device->port, SERIAL_QUEUE_INPUT);
 	}
 
 	return rc;

src/mares_darwin.c

@@ -158,6 +158,7 @@ mares_darwin_device_open (device_t **out, const char* name, unsigned int model)
 	}
 
 	// Make sure everything is in a sane state.
+	serial_sleep (100);
 	serial_flush (device->base.port, SERIAL_QUEUE_BOTH);
 
 	// Override the base class values.