Cellular: update attach test

Author: Mudassar Hussain

features/cellular/framework/AT/AT_CellularNetwork.cpp

@@ -556,10 +556,20 @@ nsapi_error_t AT_CellularNetwork::set_registration(const char *plmn)
 
     if (!plmn) {
         tr_debug("Automatic network registration");
-        _at.cmd_start("AT+COPS=0");
+        uint8_t len=8;
+        uint8_t buf[8];
+        _at.cmd_start("AT+COPS?");
         _at.cmd_stop();
         _at.resp_start();
+        _at.read_bytes(buf,len);
         _at.resp_stop();
+        if (strncmp((char*)buf,"+COPS: 0",len) != 0) {
+            _at.clear_error();
+            _at.cmd_start("AT+COPS=0");
+            _at.cmd_stop();
+            _at.resp_start();
+            _at.resp_stop();
+        }
     } else {
         tr_debug("Manual network registration to %s", plmn);
         _at.cmd_start("AT+COPS=4,2,");