drivers/ieee802154/at86rf230: replace the code under _init and _exit by macro

The code under _init and _exit functions is similar to the code of
module_spi_driver macro, which is a wrapper to the module_driver macro,
so use it instead.

Signed-off-by: Alexander Smirnov <alex.bluesman.smirnov@gmail.com>
Cc: Devendra Naga <develkernel412222@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>