mac802154: llsec: fix incorrect lock pairing

In encrypt, sec->lock is taken with read_lock_bh, so in the error path,
we must read_unlock_bh.

Signed-off-by: Phoebe Buckheister <phoebe.buckheister@itwm.fraunhofer.de>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>