Fix type of Adafruit_MQTT_Subscribe::lastread

The lastread array stores the data bytes of the most recently read
received packet. It was previously declared as an array of pointers to
uint8_t, instead of an array of uint8_t, which obviously was not the
intention. Because the examples explicitely cast to (char*) for printing
(because Print::println(uint8_t[]) is not defined) and a pointer is at
least as big as uint8_t and memcpy uses void*, this problem did not show
up before.

Adafruit_MQTT.h

@@ -202,7 +202,7 @@ class Adafruit_MQTT_Subscribe {
   const char *topic;
   uint8_t qos;
 
-  uint8_t * lastread[SUBSCRIPTIONDATALEN];
+  uint8_t lastread[SUBSCRIPTIONDATALEN];
  private:
   Adafruit_MQTT *mqtt;
 };