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: e7300d04bd08 ("MIPS: BCM63xx: Add support for the Broadcom BCM63xx family of SOCs.")
Signed-off-by: Anastasia Belova <abelova@xxxxxxxxxxxxx>