spi: sh-msiof: Drop comma after OF match table sentinel

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://patch.msgid.link/2ab5745407339ba54b63c3e6410082c7c566bf95.1747401908.git.geert+renesas@glider.be


Signed-off-by: Mark Brown <broonie@kernel.org>