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: facdf0ed ("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>