fix(modem): Fix AT client example to use custom AT processing

Need the callback reset upon removal of our custom AT command processing.
This issue has been introduced in cb6e03a when refactoring DTE
callbacks.

Closes #352

Author: david-cermak

components/esp_modem/src/esp_modem_dte.cpp

@@ -313,6 +313,7 @@ void DTE::on_read(got_line_cb on_read_cb)
     if (on_read_cb == nullptr) {
         primary_term->set_read_cb(nullptr);
         internal_lock.unlock();
+        set_command_callbacks();
         return;
     }
     internal_lock.lock();