crypto: spacc - Fix uninitialized variable in spacc_aead_process()

Smatch complains that:

    drivers/crypto/dwc-spacc/spacc_aead.c:1031 spacc_aead_process()
    error: uninitialized symbol 'ptaadsize'.

This could happen if, for example, tctx->mode was CRYPTO_MODE_NULL and
req->cryptlen was less than icvremove.

Fixes: 06af76b4 ("crypto: spacc - Add SPAcc aead support")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>