fixup! drivers: add PCF857X I/O expander driver

drivers/pcf857x/pcf857x.c

@@ -153,6 +153,13 @@ int pcf857x_gpio_init(pcf857x_t *dev, gpio_t pin, gpio_mode_t mode)
 
     DEBUG_DEV("pin=%u mode=%u", dev, pin, mode);
 
+    /*
+     * Since the LOW output is the only actively driven level possible with
+     * this expander, only in the case of GPIO_OUT we write a 0 to the pin
+     * to configure the pin as an output and actively drive it LOW. In all
+     * other modes, the pin is configured as an input and pulled-up to HIGH
+     * with the weak pull-up to emulate them.
+     */
     switch (mode) {
         case GPIO_IN_PD: DEBUG_DEV("gpio mode GPIO_IN_PD not supported", dev, mode);
                          return -PCF857X_ERROR_INV_MODE;