blackfin: bf609: let clk_disable() return immediately if clk is NULL

blackfin: bf609: let clk_disable() return immediately if clk is NULL

In many of clk_disable() implementations, it is a no-op for a NULL
pointer input, but this is one of the exceptions.

Making it treewide consistent will allow clock consumers to call
clk_disable() without NULL pointer check.

Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Masahiro Yamada <[email protected]>
Cc: Stephen Boyd <[email protected]>
Cc: Ralf Baechle <[email protected]>
Cc: Michael Turquette <[email protected]>
Cc: Steven Miao <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>