mrf24j40: async interrupt handling

This patch removes the threaded irq handling and do a hardirq instead.
We need to switch to spi_async for this step for getting the irq status
register.

Reviewed-by: Stefan Schmidt <stefan@osg.samsung.com>
Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>