xen/uart: report an error if the device type is not supported

When using key pairs for the uart configuration (com1 and com2 command line
options), report an error if the passed device=<type> is not recognized
instead of silently ignoring it.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>

xen/drivers/char/ns16550.c

@@ -1674,6 +1674,8 @@ static bool __init parse_namevalue_pairs(char *str, struct ns16550 *uart)
                 pci_uart_config(uart, 0, uart - ns16550_com);
                 dev_set = true;
             }
+            else
+                PARSE_ERR_RET("Unknown device type %s\n", param_value);
             break;
 
         case port_bdf: