mmc: host: Drop commas after SoC match table sentinels

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

Add comments to clarify the purpose of the empty elements.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Link: https://lore.kernel.org/r/9050fa278eaaa9e6ec719a3b158a2fad285560d0.1646311673.git.geert+renesas@glider.be


Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>