sh: clk: Fix clk_enable() to return 0 on NULL clk

On SH, devm_clk_get_optional_enabled() fails with -EINVAL if the clock
is not found.  This happens because __devm_clk_get() assumes it can pass
a NULL clock pointer (as returned by clk_get_optional()) to the init()
function (clk_prepare_enable() in this case), while the SH
implementation of clk_enable() considers that an error.

Fix this by making the SH clk_enable() implementation return zero
instead, like the Common Clock Framework does.
Reported-by: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Tested-by: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>
Acked-by: Stephen Boyd <sboyd@kernel.org>
Link: https://lore.kernel.org/r/b53e6b557b4240579933b3359dda335ff94ed5af.1675354849.git.geert+renesas@glider.beSigned-off-by: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>

drivers/sh/clk/core.c

@@ -295,7 +295,7 @@ int clk_enable(struct clk *clk)
 	int ret;
 
 	if (!clk)
-		return -EINVAL;
+		return 0;
 
 	spin_lock_irqsave(&clock_lock, flags);
 	ret = __clk_enable(clk);