ieee802154: cc2520: use __func__ macro for debug messages

Instead of having the function name hard-coded (it might change and we
forgot to update them in the debug output) we can use __func__ instead
and also shorter the line so we do not need to break it.
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>