cpu: stm32f3: periph: gpio: fix possibly uninitialized variable access

cpu/stm32f3/periph/gpio.c

@@ -126,6 +126,8 @@ int gpio_init_out(gpio_t dev, gpio_pp_t pushpull)
             pin = GPIO_11_PIN;
             break;
 #endif
+        default:
+            return -1;
     }
 
     port->MODER &= ~(2 << (2 * pin));           /* set pin to output mode */
@@ -229,6 +231,8 @@ int gpio_init_in(gpio_t dev, gpio_pp_t pushpull)
             pin = GPIO_11_PIN;
             break;
 #endif
+        default:
+            return -1;
     }
 
     port->MODER &= ~(3 << (2 * pin));           /* configure pin as input */
@@ -597,6 +601,8 @@ int gpio_read(gpio_t dev)
             pin = GPIO_11_PIN;
             break;
 #endif
+        default:
+            return -1;
     }
 
     if (port->MODER & (3 << (pin * 2))) {       /* if configured as output */