clk: sunxi: pll2: Fix return value check in sun4i_pll2_setup()

In case of error, the functions clk_register_composite() and
clk_register_divider() 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>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>