Revert "gpio: remove flag MRAA_GPIO_DISABLE"

This reverts commit e4ea117.

Author: nascs

src/gpio/gpio.c

@@ -1116,6 +1116,9 @@ mraa_gpio_mode(mraa_gpio_context dev, mraa_gpio_mode_t mode)
             case MRAA_GPIO_PULLDOWN:
                 flags |= GPIOHANDLE_REQUEST_BIAS_PULL_DOWN;
                 break;
+            case MRAA_GPIO_DISABLE:
+                flags |= GPIOHANDLE_REQUEST_BIAS_DISABLE;
+                break;
             default:
                 return MRAA_ERROR_FEATURE_NOT_IMPLEMENTED;
         }