crypto: sa2ul - Fix inconsistent IS_ERR and PTR_ERR

Fix inconsistent IS_ERR and PTR_ERR in sa_dma_init(). The proper pointer
to be passed as argument to PTR_ERR() is dd->dma_tx.

This bug was detected with the help of Coccinelle.

Fixes: 7694b6ca ("crypto: sa2ul - Add crypto driver")
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>