ARM: mvebu: fix return value check in armada_xp_pmsu_cpufreq_init()

In case of error, the function clk_get() returns ERR_PTR()
and never returns NULL. The NULL test in the return value
check should be replaced with IS_ERR().

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Link: https://lkml.kernel.org/r/1406038688-26417-1-git-send-email-weiyj_lk@163.com


Signed-off-by: Jason Cooper <jason@lakedaemon.net>