dma-buf: Cleanup comments on dma_buf_map_attachment()

Mappings need to be unmapped by calling dma_buf_unmap_attachment() and
not by calling again dma_buf_map_attachment(). Also fix some spelling
mistakes.

Signed-off-by: Liviu Dudau <liviu.dudau@arm.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171101140630.2884-1-Liviu.Dudau@arm.com