drivers: serial: ns16550: allow clock_control_on returning ENOSYS

Fixes a regression caused by: 88830a3
The ns16550 driver should handle possibility of
clock_control_on being a stub (i.e. returning ENOSYS)

Signed-off-by: Dominik Lau <[email protected]>
(cherry picked from commit b4c0ec5)

Author: laudominik

drivers/serial/uart_ns16550.c

@@ -621,7 +621,7 @@ static int uart_ns16550_configure(const struct device *dev,
 		}
 
 		ret = clock_control_on(dev_cfg->clock_dev, dev_cfg->clock_subsys);
-		if (ret != 0 && ret != -EALREADY) {
+		if (ret != 0 && ret != -EALREADY && ret != -ENOSYS) {
 			goto out;
 		}