dma-buf: remove dma_buf_debugfs_create_file()

There is only a single user of dma_buf_debugfs_create_file() and that
one got the function pointer cast wrong. With that one fixed, there is
no need to have a wrapper for debugfs_create_file(), just call it
directly.

With no users left, we can remove dma_buf_debugfs_create_file().

While at it, simplify the error handling in dma_buf_init_debugfs()
slightly.

Signed-off-by: Mathias Krause <minipli@googlemail.com>
Cc: Sumit Semwal <sumit.semwal@linaro.org>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Sumit Semwal <sumit.semwal@linaro.org>
Link: http://patchwork.freedesktop.org/patch/msgid/1466339491-12639-2-git-send-email-minipli@googlemail.com