serial: stm32: avoid kernel warning on absence of optional IRQ

stm32_init_port() of the stm32-usart may trigger a warning in
platform_get_irq() when the device tree specifies no wakeup interrupt.

The wakeup interrupt is usually a board-specific GPIO and the driver
functions correctly in its absence. The mainline stm32mp151.dtsi does
not specify it, so all mainline device trees trigger an unnecessary
kernel warning. Use of platform_get_irq_optional() avoids this.

Fixes: 2c58e560 ("serial: stm32: fix the get_irq error case")
Signed-off-by: Holger Assmann <h.assmann@pengutronix.de>
Cc: stable <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/20200813152757.32751-1-h.assmann@pengutronix.deSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/stm32-usart.c

@@ -970,7 +970,7 @@ static int stm32_init_port(struct stm32_port *stm32port,
 		return ret;
 
 	if (stm32port->info->cfg.has_wakeup) {
-		stm32port->wakeirq = platform_get_irq(pdev, 1);
+		stm32port->wakeirq = platform_get_irq_optional(pdev, 1);
 		if (stm32port->wakeirq <= 0 && stm32port->wakeirq != -ENXIO)
 			return stm32port->wakeirq ? : -ENODEV;
 	}