cypto: mediatek - fix leaks in mtk_desc_ring_alloc

In the init loop, if an error occurs in function 'dma_alloc_coherent',
then goto the err_cleanup section, after run i--,
in the array ring, the struct mtk_ring with index i will not be released,
causing memory leaks

Fixes: 785e5c61

 ("crypto: mediatek - Add crypto driver support for some MediaTek chips")
Cc: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: Xiaoliang Pang <dawning.pang@gmail.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>