thunderbolt: nhi: Fix kernel-doc descriptions of non-static functions

Fix kernel-doc descriptions of the two non-static functions. This also
gets rids of the warnings on W=1 build.

Reported-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Reviewed-by: Lee Jones <lee.jones@linaro.org>