It does not make sense to have a comma after a sentinel, as any new
elements must be added before the sentinel.

Add a comment to clarify the purpose of the empty element.
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/0b8ad4dcc185aa7a17655983e0eb5690d8fed460.1646311558.git.geert+renesas@glider.beSigned-off-by: Vinod Koul <vkoul@kernel.org>