Fix packet byte datatype (#1)

On bk7231t, having currentByte as a char causes a problem when receiving packet types >= 8 (in the packet the type is in the high nibble, ie 0x80), when byte shifting >> 4 they get sign extended so we end up with packet type 0xf9 instead of 0x9 for suback.

src/AsyncMqttClient.cpp

@@ -393,7 +393,7 @@ void AsyncMqttClient::_onAck(AsyncClient* client, size_t len, uint32_t time) {
 void AsyncMqttClient::_onData(AsyncClient* client, char* data, size_t len) {
   (void)client;
   size_t currentBytePosition = 0;
-  char currentByte;
+  uint8_t currentByte;
   do {
     switch (_parsingInformation.bufferState) {
       case AsyncMqttClientInternals::BufferState::NONE:
@@ -895,4 +895,4 @@ uint16_t AsyncMqttClient::publish(const char* topic, uint8_t qos, bool retain, c
 
 const char* AsyncMqttClient::getClientId() {
   return _clientId;
-}
+}