samples: drivers: crypto: Print correct buffer comparison on failure

When testing whether the CTR mode decrypted the payload properly, a
comparison of `decrypt.out_buf` with the known plain text `plaintext`
is performed, but the buffer comparison that is printed uses
`plaintext` and `encrypt.out_buf` instead.

Coverity-CID: 181847

Signed-off-by: Leandro Pereira <leandro.pereira@intel.com>