mrf24j40: fix security-enabled processing on inbound frames

When receiving a security-enabled IEEE 802.15.4 frame, the MRF24J40
triggers a SECIF interrupt that needs to be handled for RX processing
to keep functioning properly.

This patch enables the SECIF interrupt and makes the MRF ignores all
hardware processing of security-enabled frames, that is handled by the
ieee802154 stack instead.

Signed-off-by: Alexander Aring <aar@pengutronix.de>
Signed-off-by: Alexandre Macabies <web+oss@zopieux.com>
Reviewed-by: Stefan Schmidt <stefan@osg.samsung.com>
Acked-by: Alan Ott <alan@signal11.us>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>