crypto: amcc - remove memset after dma_alloc_coherent

In commit af7ddd8a


("Merge tag 'dma-mapping-4.21' of git://git.infradead.org/users/hch/dma-mapping"),
dma_alloc_coherent has already zeroed the memory.
So memset is not needed.

Signed-off-by: Fuqian Huang <huangfq.daxian@gmail.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>