cc2520: set the default fifo pin value from platform data

When the device tree support is disabled, the fifo_pin is uninitialized,
this patch will set the fifo_pin value based on platform data

Signed-off-by: Yong Li <sdliyong@gmail.com>
Acked-by: Varka Bhadram <varkabhadram@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>