clk: clk-prima2: fix return value check in prima2_clk_init()

clk: clk-prima2: fix return value check in prima2_clk_init()

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

Signed-off-by: Xu Wang <vulab@iscas.ac.cn>
Link: https://lore.kernel.org/r/20200921034522.9077-1-vulab@iscas.ac.cn
Acked-by: Barry Song <baohua@kernel.org>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>