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>

drivers/thunderbolt/nhi.c

@@ -592,6 +592,7 @@ EXPORT_SYMBOL_GPL(tb_ring_alloc_rx);
 
 /**
  * tb_ring_start() - enable a ring
+ * @ring: Ring to start
  *
  * Must not be invoked in parallel with tb_ring_stop().
  */
@@ -667,6 +668,7 @@ EXPORT_SYMBOL_GPL(tb_ring_start);
 
 /**
  * tb_ring_stop() - shutdown a ring
+ * @ring: Ring to stop
  *
  * Must not be invoked from a callback.
  *