sh: Fix clock framework compiler warnings.

  CC      arch/sh/kernel/cpu/clock.o
  arch/sh/kernel/cpu/clock.c: In function 'clk_disable':
  arch/sh/kernel/cpu/clock.c:156: warning: 'return' with a value, in function returning void

Introduced by ("sh: enable and disable clocks recursively").

Signed-off-by: Paul Mundt <lethal@linux-sh.org>