HID: Intel-thc-hid: Intel-thc: Add thc_dma_content into kernel doc

Convert thc_dma_content structure comments format into kernel doc
format, let it can be included into kernel doc.

Signed-off-by: Even Xu <even.xu@intel.com>
Signed-off-by: Jiri Kosina <jkosina@suse.com>