watchdog: lenovo_se30_wdt: Fix possible devm_ioremap() NULL pointer dereference in lenovo_se30_wdt_probe()

In the Linux kernel, the following vulnerability has been resolved: watchdog: lenovo_se30_wdt: Fix possible devm_ioremap() NULL pointer dereference in lenovo_se30_wdt_probe() devm_ioremap() returns NULL on error. Currently, lenovo_se30_wdt_probe() does not check for this case, which results in a NULL pointer dereference. Add NULL check after devm_ioremap() to prevent this issue.