staging: ti-soc-thermal: remove kernel-doc warnings reported with -v

Fix all kernel-doc warnings by add documentation about returns.

Signed-off-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Eduardo Valentin <eduardo.valentin@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>