Cellular: Fix SIM pin enter command

Author: Ari Parkkila

features/cellular/framework/AT/AT_CellularDevice.cpp

@@ -241,7 +241,7 @@ nsapi_error_t AT_CellularDevice::set_pin(const char *sim_pin)
     const bool stored_debug_state = _at->get_debug();
     _at->set_debug(false);
 
-    _at->at_cmd_discard("+CPIN", "=,", "%s", sim_pin);
+    _at->at_cmd_discard("+CPIN", "=", "%s", sim_pin);
 
     _at->set_debug(stored_debug_state);