crypto: aead - fixed style error in aead.c

Fixed style error identified by checkpatch.

ERROR: do not use assignment in if condition
+       if ((err = crypto_register_instance(tmpl, inst))) {

Signed-off-by: Joshua I. James <joshua@cybercrimetech.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>