clk: fix return value check in of_fixed_clk_setup()

In case of error, the function clk_register_fixed_rate() returns
ERR_PTR() not NULL pointer. The NULL test in the return value
check should be replaced with IS_ERR().

dpatch engine is used to auto generated this patch.
(https://github.com/weiyj/dpatch

)

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Mike Turquette <mturquette@linaro.org>