clk: Ignore error and NULL pointers passed to clk_{unprepare, disable}()

This simplifies error paths in drivers that use optional clocks
by allowing the NULL or error pointer to be passed
unconditionally.

Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Mike Turquette <mturquette@linaro.org>

diff --cc drivers/clk/clk.c
Simple merge