crypto: sunxi-ss - Fix a possible driver hang with ciphers

The sun4i_ss_opti_poll function cipher data until the output miter have
a length of 0.
If the crypto API client, give more SGs than necessary this could result
in an infinite loop.
Fix it by checking for remaining bytes, just like sun4i_ss_cipher_poll().

Signed-off-by: LABBE Corentin <clabbe.montjoie@gmail.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>