serial: Fix potential null-ptr-deref in mlb_usio_probe()

In the Linux kernel, the following vulnerability has been resolved: serial: Fix potential null-ptr-deref in mlb_usio_probe() devm_ioremap() can return NULL on error. Currently, mlb_usio_probe() does not check for this case, which could result in a NULL pointer dereference. Add NULL check after devm_ioremap() to prevent this issue.