In uart_cnprobe(), fill in the cn_name field of the consdev structure.

The core console code checks this field when a console is added and
emits a warning if it's empty. In practice the warning is harmless for
uart(4), because the cn_name is filled in as soon as the device name is
known; which is when the device is enumerated.
To avoid the warning, to avoid possible complications caused by emitting
the warning without there (possibly) being a console selected yet and to
avoid complications when the UART isn't found during bus enumeration, we
just preset the cn_name field here to the name of the driver.

sys/dev/uart/uart_tty.c

@@ -76,6 +76,7 @@ uart_cnprobe(struct consdev *cp)
 	if (uart_probe(&uart_console))
 		return;
 
+	strlcpy(cp->cn_name, uart_driver_name, sizeof(cp->cn_name));
 	cp->cn_pri = (boothowto & RB_SERIAL) ? CN_REMOTE : CN_NORMAL;
 	cp->cn_arg = &uart_console;
 }