hwspinlock: fix return value check in stm32_hwspinlock_probe()

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

Fixes: f24fcff1 ("hwspinlock: add STM32 hwspinlock device")
Acked-by: Benjamin Gaignard <benjamin.gaignard@gmail.com>
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>