PNP: Do not disable devices on suspend when they cannot be re-enabled on resume

On an Advantech MICA-071 tablet, with a builtin barcode scanner connected
to ttyS0, the following message is shown on suspend:

serial 00:02: disabled

And after suspend/resume trying to use the barcode scanner / ttyS0 shows:

serial 00:02: LSR safety check engaged!

Indicating that the UARTs io-ports are no longer reachable.

This is caused by __pnp_bus_suspend() calling pnp_stop_dev() on the "00:02"
pnp device on suspend (this outputs the disabled message).

The problem is that pnp_can_write() returns false for the "00:02" pnp
device, so after disabling it (disabling its decoding of IO addresses)
during suspend, it cannot be re-enabled.

Add a pnp_can_write() check to the suspend path and only disable devices
which can actually be re-enabled on resume.

This fixes the Advantech MICA-071's ttyS0 no longer working after
a suspend/resume.
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

drivers/pnp/driver.c

@@ -182,7 +182,8 @@ static int __pnp_bus_suspend(struct device *dev, pm_message_t state)
 			return error;
 	}
 
-	if (pnp_can_disable(pnp_dev)) {
+	/* can_write is necessary to be able to re-start the device on resume */
+	if (pnp_can_disable(pnp_dev) && pnp_can_write(pnp_dev)) {
 		error = pnp_stop_dev(pnp_dev);
 		if (error)
 			return error;