s390/crypto: fix aes_s390 crypto module unload problem

If a machine has no hardware support for the xts-aes or ctr-aes algorithms
they are not registered in aes_s390_init. But aes_s390_fini unconditionally
unregisters the algorithms which causes crypto_remove_alg to crash.
Add two flag variables to remember if xts-aes and ctr-aes have been added.

Signed-off-by: Ingo Tuchscherer <ingo.tuchscherer@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>