serial: core: Fix regression when runtime PM is not enabled

Commit 45a3a8e ("serial: core: Revert checks for tx runtime PM state")
caused a regression for Sun Ultra 60 for the sunsab driver as reported by
Nick Bowler <[email protected]>.

We need to add back the check runtime PM enabled state for serial port
controller device, I wrongly assumed earlier we could just remove it.

Fixes: 45a3a8e ("serial: core: Revert checks for tx runtime PM state")
Cc: stable <[email protected]>
Reported-by: Nick Bowler <[email protected]>
Signed-off-by: Tony Lindgren <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Greg Kroah-Hartman <[email protected]>

Author: tmlind

drivers/tty/serial/serial_core.c

@@ -156,7 +156,7 @@ static void __uart_start(struct uart_state *state)
 	 * enabled, serial_port_runtime_resume() calls start_tx() again
 	 * after enabling the device.
 	 */
-	if (pm_runtime_active(&port_dev->dev))
+	if (!pm_runtime_enabled(port->dev) || pm_runtime_active(&port_dev->dev))
 		port->ops->start_tx(port);
 	pm_runtime_mark_last_busy(&port_dev->dev);
 	pm_runtime_put_autosuspend(&port_dev->dev);