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>
mac802154: llsec: fold useless return value check

llsec_do_encrypt will never return a positive value, so the restriction
to 0-or-negative on return is useless.

Signed-off-by: Phoebe Buckheister <phoebe.buckheister@itwm.fraunhofer.de>
mac802154: llsec: correctly lookup implicit-indexed keys

Key id comparison for type 1 keys (implicit source, with index) should
return true if mode and id are equal, not false.

Signed-off-by: Phoebe Buckheister <phoebe.buckheister@itwm.fraunhofer.de>