bluetooth: hci: spi: Fix compile warning

drivers/bluetooth/hci/spi.c: In function ‘bt_spi_open’:
drivers/bluetooth/hci/spi.c:475:31: error: passing argument 2 of ‘gpio_init_callback’ from incompatible pointer type [-Werror=incompatible-pointer-types]
  gpio_init_callback(&gpio_cb, bt_spi_isr, BIT(GPIO_IRQ_PIN));
                               ^~~~~~~~~~
In file included from drivers/bluetooth/hci/spi.c:9:0:
include/gpio.h:247:20: note: expected ‘gpio_callback_handler_t {aka void (*)(struct device *, struct gpio_callback *, long unsigned int)}’ but argument is of type ‘void (*)(struct device *, struct gpio_callback *, unsigned int)’
 static inline void gpio_init_callback(struct gpio_callback *callback,
                    ^~~~~~~~~~~~~~~~~~

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>