Nordic BLE: Improve return of nRF5xn::getGap.

Return the derived type instead of the abstract one. This is legal as C++ supports covariant returns.

Author: pan-

features/FEATURE_BLE/targets/TARGET_NORDIC/TARGET_NRF51/source/nRF5xn.h

@@ -55,7 +55,7 @@ class nRF5xn : public BLEInstanceBase
      *        always needed in a BLE application. Therefore it is allocated
      *        statically.
      */
-    virtual Gap &getGap() {
+    virtual nRF5xGap &getGap() {
         return gapInstance;
     };
 

features/FEATURE_BLE/targets/TARGET_NORDIC/TARGET_NRF52/source/nRF5xn.h

@@ -54,7 +54,7 @@ class nRF5xn : public BLEInstanceBase
      *        always needed in a BLE application. Therefore it is allocated
      *        statically.
      */
-    virtual Gap &getGap() {
+    virtual nRF5xGap &getGap() {
         return gapInstance;
     };