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

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.

Fixes: ba44dc043004 ("serial: Add Milbeaut serial control")
Signed-off-by: Henry Martin <bsdhenrymartin@gmail.com>
Link: https://lore.kernel.org/r/20250403070339.64990-1-bsdhenrymartin@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>