target: UBLOX: Fix build warning in UBLOX_AT_CellularNetwork.cpp

Fix this build warning seen when building with ARMCC

Compile [ 13.7%]: UBLOX_AT_CellularNetwork.cpp
[Warning] UBLOX_AT_CellularNetwork.cpp@65,0:  #111-D: statement is unreachable

features/cellular/framework/targets/UBLOX/AT/UBLOX_AT_CellularNetwork.cpp

@@ -38,6 +38,8 @@ AT_CellularNetwork::RegistrationMode UBLOX_AT_CellularNetwork::has_registration(
 
 nsapi_error_t UBLOX_AT_CellularNetwork::set_access_technology_impl(RadioAccessTechnology opRat)
 {
+    nsapi_error_t ret = NSAPI_ERROR_OK;
+
     switch (opRat) {
 #if defined(TARGET_UBLOX_C030_U201) || defined(TARGET_UBLOX_C027)
         case RAT_GSM:
@@ -63,9 +65,9 @@ nsapi_error_t UBLOX_AT_CellularNetwork::set_access_technology_impl(RadioAccessTe
 #endif
         default: {
             _op_act = RAT_UNKNOWN;
-            return NSAPI_ERROR_UNSUPPORTED;
+            ret = NSAPI_ERROR_UNSUPPORTED;
         }
     }
 
-    return NSAPI_ERROR_OK;
+    return(ret);
 }