AS923: Using LoRa modulation comatible DR as default

DR7 is used for FSK in AS932 region. As a default max DR, we should use a LoRa modulation
compatible data rate. Ofcourse if a device wishes to use FSK, it can set DR7 using set_data_rate() API
and turning off ADR, or an NS can configure a new channel for the device utilizing DR7.

Author: Hasnain Virk

features/lorawan/lorastack/phy/LoRaPHYAS923.cpp

@@ -67,7 +67,7 @@
  */
 #define AS923_DEFAULT_DATARATE                      DR_2
 
-#define AS923_DEFAULT_MAX_DATARATE                  DR_7
+#define AS923_DEFAULT_MAX_DATARATE                  DR_5
 
 /*!
  * The minimum datarate which is used when the