adc: add close adc func

Change-Id: I02fa5c283a191335a5fe477d89a0388daff26109
diff --git a/mbtk/mbtk_lib/src/mbtk_gpio.c b/mbtk/mbtk_lib/src/mbtk_gpio.c
index 15b8312..8b7396a 100755
--- a/mbtk/mbtk_lib/src/mbtk_gpio.c
+++ b/mbtk/mbtk_lib/src/mbtk_gpio.c
@@ -568,5 +568,6 @@
             //printf("############gpio [%d] test success############\n", test_gpio[i]);
         }
     }
+    mbtk_adc_close();
     return n;
 }