m68k: let clk_enable() return immediately if clk is NULL

Similar to commit<742859ad>("m68k: let clk_disable() return immediately if clk is NULL").
there should be a check for clk to prevent NULL pointer dereference.

Signed-off-by: Defang Bo <bodefang@126.com>
Signed-off-by: Greg Ungerer <gerg@linux-m68k.org>