clk: sp7021: fix return value check in sp7021_clk_probe()

clk: sp7021: fix return value check in sp7021_clk_probe()

devm_platform_ioremap_resource() never returns NULL pointer,
it will return ERR_PTR() when it fails, so replace the check
with IS_ERR().

Fixes: d54c1fd4a51e ("clk: Add Sunplus SP7021 clock driver")
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Link: https://lore.kernel.org/r/20231128133016.2494699-1-yangyingliang@huawei.com
Signed-off-by: Stephen Boyd <sboyd@kernel.org>