AP_GPS: stop reading arbitrary amounts of data in GPS init

We don't like doing this in other places as the vehicle can go out to
lunch for arbitrary periods of time

libraries/AP_GPS/AP_GPS.cpp

@@ -744,7 +744,14 @@ AP_GPS_Backend *AP_GPS::_detect_instance(uint8_t instance)
         send_blob_update(instance);
     }
 
-    while (initblob_state[instance].remaining == 0 && _port[instance]->available() > 0) {
+    if (initblob_state[instance].remaining != 0) {
+        // don't run detection engines if we haven't sent out the initblobs
+        return nullptr;
+    }
+
+    uint16_t bytecount = MIN(8192U, _port[instance]->available());
+
+    while (bytecount-- > 0) {
         uint8_t data = _port[instance]->read();
         /*
           running a uBlox at less than 38400 will lead to packet