ieee802154: cc2520: switch from BUG_ON() to WARN_ON() on problem

The check is valid but it does not warrant to crash the kernel. A
WARN_ON() is good enough here.
Found by checkpatch.

Signed-off-by: Stefan Schmidt <stefan@osg.samsung.com>
Acked-by: Varka Bhadram <varkabhadram@gmail.com>
Signed-off-by: Stefan Schmidt <stefan@osg.samsung.com>