serial: exar: Leave MPIOs as output for Commtech adapters

Commtech adapters apparently need the original setting as outputs, see
https://marc.info/?l=linux-gpio&m=149557425201323&w=2. Account for that.

Fixes: 7dea8165f1d6 ("serial: exar: Preconfigure xr17v35x MPIOs as output")
Signed-off-by: Jan Kiszka <[e-mail]>
Reviewed-by: Andy Shevchenko <[e-mail]>
Reviewed-by: Linus Walleij <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/tty/serial/8250/8250_exar.c

···
 	return default_setup(priv, pcidev, idx, offset, port);
 }
 
-static void setup_gpio(u8 __iomem *p)
+static void setup_gpio(struct pci_dev *pcidev, u8 __iomem *p)
 {
+	/*
+	 * The Commtech adapters required the MPIOs to be driven low. The Exar
+	 * devices will export them as GPIOs, so we pre-configure them safely
+	 * as inputs.
+	 */
+	u8 dir = pcidev->vendor == PCI_VENDOR_ID_EXAR ? 0xff : 0x00;
+
 	writeb(0x00, p + UART_EXAR_MPIOINT_7_0);
 	writeb(0x00, p + UART_EXAR_MPIOLVL_7_0);
 	writeb(0x00, p + UART_EXAR_MPIO3T_7_0);
 	writeb(0x00, p + UART_EXAR_MPIOINV_7_0);
-	writeb(0xff, p + UART_EXAR_MPIOSEL_7_0);
+	writeb(dir,  p + UART_EXAR_MPIOSEL_7_0);
 	writeb(0x00, p + UART_EXAR_MPIOOD_7_0);
 	writeb(0x00, p + UART_EXAR_MPIOINT_15_8);
 	writeb(0x00, p + UART_EXAR_MPIOLVL_15_8);
 	writeb(0x00, p + UART_EXAR_MPIO3T_15_8);
 	writeb(0x00, p + UART_EXAR_MPIOINV_15_8);
-	writeb(0xff, p + UART_EXAR_MPIOSEL_15_8);
+	writeb(dir,  p + UART_EXAR_MPIOSEL_15_8);
 	writeb(0x00, p + UART_EXAR_MPIOOD_15_8);
 }
 
···
 
 	if (idx == 0) {
 		/* Setup Multipurpose Input/Output pins. */
-		setup_gpio(p);
+		setup_gpio(pcidev, p);
 
 		port->port.private_data = xr17v35x_register_gpio(pcidev);
 	}