crypto: mediatek - Fix wrong return value in mtk_desc_ring_alloc()

In case of memory allocation failure, a negative error code should
be returned.

Fixes: 785e5c61 ("crypto: mediatek - Add crypto driver support for some MediaTek chips")
Cc: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: Tianjia Zhang <tianjia.zhang@linux.alibaba.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>