mraa.c: add syslog msg in case we get an unknown pinmode

Signed-off-by: Brendan Le Foll <brendan.le.foll@intel.com>

src/mraa.c

@@ -263,7 +263,9 @@ mraa_pin_mode_test(int pin, mraa_pinmodes_t mode)
         case MRAA_PIN_UART:
             if (plat->pins[pin].capabilites.uart == 1)
                 return 1;
-        default: break;
+        default:
+            syslog(LOG_NOTICE, "requested pinmode invalid");
+            break;
     }
     return 0;
 }