clk: renesas: cpg-mssr: Initialize error pointer using ERR_PTR()

Coccinelle warns:

    drivers/clk/renesas/renesas-cpg-mssr.c:323:14-21: ERROR: PTR_ERR applied after initialization to constant on line 260

Initialize clk using ERR_PTR(-ENOTSUPP) instead of NULL to fix this.

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>