PNP: add interface to retrieve ACPI device from a PNPACPI device

Add pnp_acpi_device(pnp_dev), which takes a PNP device and returns the
associated ACPI device (or NULL, if the device is not a PNPACPI device).

This allows us to write a PNP driver that can manage both traditional
PNPBIOS and ACPI devices, treating ACPI-only functionality as an optional
extension.

Signed-off-by: Bjorn Helgaas <bjorn.helgaas@hp.com>
Signed-off-by: Len Brown <len.brown@intel.com>
diff --git a/drivers/pnp/pnpacpi/core.c b/drivers/pnp/pnpacpi/core.c
index b2348fc..5314bf6 100644
--- a/drivers/pnp/pnpacpi/core.c
+++ b/drivers/pnp/pnpacpi/core.c
@@ -144,7 +144,7 @@
 }
 #endif
 
-static struct pnp_protocol pnpacpi_protocol = {
+struct pnp_protocol pnpacpi_protocol = {
 	.name	 = "Plug and Play ACPI",
 	.get	 = pnpacpi_get_resources,
 	.set	 = pnpacpi_set_resources,
@@ -154,6 +154,7 @@
 	.resume = pnpacpi_resume,
 #endif
 };
+EXPORT_SYMBOL(pnpacpi_protocol);
 
 static int __init pnpacpi_add_device(struct acpi_device *device)
 {