clocksource/drivers/ingenic_ost: Fix return value check in ingenic_ost_probe()

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

Fixes: ca7b72b5 ("clocksource: Add driver for the Ingenic JZ47xx OST")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20210308123031.2285083-1-weiyongjun1@huawei.com