m68k: allow NULL clock for clk_get_rate

Make the behaviour of clk_get_rate consistent with common clk's
clk_get_rate by accepting NULL clocks as parameter. Some device
drivers rely on this, and will cause an OOPS otherwise.

Fixes: facdf0ed4f59 ("m68knommu: introduce basic clk infrastructure")
Cc: Greg Ungerer <gerg@linux-m68k.org>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: linux-m68k@lists.linux-m68k.org
Cc: linux-kernel@vger.kernel.org
Reported-by: Mathias Kresin <dev@kresin.me>
Signed-off-by: Jonas Gorski <jonas.gorski@gmail.com>
Signed-off-by: Greg Ungerer <gerg@linux-m68k.org>

arch/m68k/coldfire/clk.c

@@ -121,6 +121,9 @@ EXPORT_SYMBOL(clk_put);
 
 unsigned long clk_get_rate(struct clk *clk)
 {
+	if (!clk)
+		return 0;
+
 	return clk->rate;
 }
 EXPORT_SYMBOL(clk_get_rate);