Merge pull request #2820 from haukepetersen/fix_stm32f1_gpioint

cpu/stm32f1: fixed GPIO interrupt cb arg handling

cpu/stm32f1/periph/gpio.c

@@ -483,6 +483,7 @@ int gpio_init_int(gpio_t dev, gpio_pp_t pullup, gpio_flank_t flank, gpio_cb_t cb
 
     /* set callback */
     config[gpio_irq].cb = cb;
+    config[gpio_irq].arg = arg;
 
     /* configure the event that triggers an interrupt */
     switch (flank) {