MIPS: BCM63xx: Let clk_disable() return immediately if clk is NULL

MIPS: BCM63xx: 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.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Cc: Stephen Boyd <sboyd@codeaurora.org>
Cc: Michael Turquette <mturquette@baylibre.com>
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org
Cc: bcm-kernel-feedback-list@broadcom.com
Cc: linux-arm-kernel@lists.infradead.org
Patchwork: https://patchwork.linux-mips.org/patch/14264/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>