hash: fix a memory leak

memalign() returns a pointer which is to be freed by free(). To call
unmap_sysmem() is incorrect, furthermore it was called in a wrong scope.

Also add a check for allocation error.

Fixes: d7af2baa ("crypto/fsl: Fix HW accelerated hash commands")
Cc: Breno Lima <breno.lima@nxp.com>
Signed-off-by: Sergei Antonov <saproj@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>