rpmsg: glink: fix some kerneldoc comments

The kerneldoc comments for the do_cleanup_msg and cleanup_done_msg
structures describe the fields, but don't prefix the field names
with "@".  Add those, to get rid of some W=1 build warnings on
an x86_64 architecture build.

Signed-off-by: Alex Elder <elder@linaro.org>
Link: https://lore.kernel.org/r/20210105235603.32663-1-elder@linaro.org


Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>