MIPS: BCM63xx: Add check for NULL for clk in clk_enable

Check clk for NULL before calling clk_enable_unlocked where clk
is dereferenced. There is such check in other implementations
of clk_enable.

Found by Linux Verification Center (linuxtesting.org) with SVACE.

Fixes: e7300d04

 ("MIPS: BCM63xx: Add support for the Broadcom BCM63xx family of SOCs.")
Signed-off-by: Anastasia Belova <abelova@astralinux.ru>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>