frdm-kw41z: Use SAUL_GPIO_INVERT on SW3, SW4 inputs

with this flag set, reading 1 means that the button is being pressed.

boards/frdm-kw41z/include/gpio_params.h

@@ -64,11 +64,13 @@ static const  saul_gpio_params_t saul_gpio_params[] =
         .name = "SW3",
         .pin = BTN0_PIN,
         .mode = BTN0_MODE,
+        .flags = (SAUL_GPIO_INVERTED),
     },
     {
         .name = "SW4",
         .pin = BTN1_PIN,
         .mode = BTN1_MODE,
+        .flags = (SAUL_GPIO_INVERTED),
     },
 };