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.