crypto: arm64/aes - do not call crypto_unregister_skcipher twice on error

When a cipher fails to register in aes_init(), the error path goes thought
aes_exit() then crypto_unregister_skciphers().
Since aes_exit calls also crypto_unregister_skcipher, this triggers a
refcount_t: underflow; use-after-free.

Signed-off-by: Corentin Labbe <clabbe@baylibre.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>