crypto: cmac - return -EINVAL if block size is unsupported

cmac_create() previously returned 0 if a cipher with a block size other
than 8 or 16 bytes was specified.  It should return -EINVAL instead.
Granted, this doesn't actually change any behavior because cryptomgr
currently ignores any return value other than -EAGAIN from template
->create() functions.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>