ACPI: PCI: Fix device reference counting in acpi_get_pci_dev()

Commit 63f534b8 ("ACPI: PCI: Rework acpi_get_pci_dev()") failed
to reference count the device returned by acpi_get_pci_dev() as
expected by its callers which in some cases may cause device objects
to be dropped prematurely.

Add the missing get_device() to acpi_get_pci_dev().

Fixes: 63f534b8 ("ACPI: PCI: Rework acpi_get_pci_dev()")
Reported-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>