Add a small delay before retrying a packet.

If the first attempt fails, that might indicate the device isn't ready
yet to service requests. In that case immediately retrying again isn't
the right solution. Adding a small delay seems to increase the success
rate, so it's a good idea anyway, regardless of the underlying reason.

src/mares_common.c

@@ -201,6 +201,7 @@ mares_common_transfer (mares_common_device_t *device, const unsigned char comman
 			return rc;
 
 		// Discard any garbage bytes.
+		serial_sleep (device->port, 100);
 		serial_flush (device->port, SERIAL_QUEUE_INPUT);
 	}