Fix NRF51, NRF52 gpio_irq_init() function

Acording to the description in `gpio_irq_hal_api.h` file `gpio_irq_init()` should return 0 on success.
Currently, it returns 1 causing the FPGA test to fail.

Author: mprse

targets/TARGET_NORDIC/TARGET_NRF5x/TARGET_NRF51/gpio_api.c

@@ -216,7 +216,7 @@ int gpio_irq_init(gpio_irq_t *obj, PinName pin, gpio_irq_handler handler, uint32
     m_channel_ids[pin] = id;
 
     gpio_apply_config(pin);
-    return 1;
+    return 0;
 }
 
 

targets/TARGET_NORDIC/TARGET_NRF5x/TARGET_NRF52/gpio_api.c

@@ -217,7 +217,7 @@ int gpio_irq_init(gpio_irq_t *obj, PinName pin, gpio_irq_handler handler, uint32
     m_channel_ids[pin] = id;
 
     gpio_apply_config(pin);
-    return 1;
+    return 0;
 }