clk: renesas: cpg-mssr: Use [] to denote a flexible array member

Flexible array members should be denoted using [] instead of [0], else
gcc will not warn when they are no longer at the end of the structure.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>