media: imx-jpeg: Remove unnecessary memset() after dma_alloc_coherent()

The `dma_alloc_coherent()' already zeroes out memory for us, so we don't
need the redundant memset().

Signed-off-by: Jason Wang <wangborong@cdjrlc.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>