s390/crypto: unlock on error in prng_tdes_read()

We added some new locking but forgot to unlock on error.

Fixes: 57127645 ("s390/zcrypt: Introduce new SHA-512 based Pseudo Random Generator.")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>