spi: sh-msiof: Do not use C++ style comment

4286db84 ("spi: sh-msiof: Add R-Car Gen 2 and 3 fallback bindings")
added a C++ style comment. This is not in keeping with the style used
for comments elsewhere in this fine. Update it accordingly.

Signed-off-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Mark Brown <broonie@kernel.org>