dma-buf: Return error instead of using a goto statement when possible

Remove an error label in dma_buf_attach() that just returns an error
code.

Change-Id: Ic11428aa1e5339f381e1219a0213302a87aa6936
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Sumit Semwal <sumit.semwal@linaro.org>