bladeRF-flash: cast callback fn to proper type

Fixes werrors:

/bladerf/bladeRF/host/utilities/bladeRF-flash/src/main.c:240:13:
    error: passing argument 7 of 'libusb_hotplug_register_callback'
    from incompatible pointer type [-Werror]
/usr/i686-w64-mingw32/include/libusb-1.0/libusb.h:1928:17:
    note: expected 'libusb_hotplug_callback_fn' but argument is of type
    'int (*)(struct libusb_context *, struct libusb_device *, enum libusb_hotplug_event,  void *)'