[rfxcom] Convert unsigned byte to int properly for message length (#10830)

Without this, any message over 127 bytes reports a negative length and
fails the message start test, which puts the code out of sync with the
rfxcom leading to lots of errors, and eventually, complete loss of
connection.

Signed-off-by: James Hewitt <james.hewitt@uk.ibm.com>

bundles/org.openhab.binding.rfxcom/src/main/java/org/openhab/binding/rfxcom/internal/connector/RFXComStreamReader.java

@@ -62,7 +62,7 @@ public class RFXComStreamReader extends Thread {
             while (!Thread.interrupted()) {
                 // First byte tells us how long the packet is
                 int bytesRead = connector.read(buf, 0, 1);
-                int packetLength = buf[0];
+                int packetLength = Byte.toUnsignedInt(buf[0]);
 
                 if (bytesRead > 0 && packetLength > 0) {
                     logger.trace("Message length is {} bytes", packetLength);