dma: mv_xor: Fix incorrect error path

Return directly if memory allocation fails. There is no need
of dma_free_coherent().

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Cc: Saeed Bishara <saeed@marvell.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>