clk: prima2: fix return value check in sirfsoc_of_clk_init()

In case of error, the function clk_get() returns ERR_PTR()
not 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>
Acked-by: Barry Song <21cnbao@gmail.com>
Signed-off-by: Mike Turquette <mturquette@linaro.org>
[mturquette@linaro.org: added missing parenthesis to fix compile break]