pci-acpi: remove unused variable in __pci_osc_support_set

The 'retval' variable in __pci_osc_support_set() is no longer
used. Remove this unused variable.

Signed-off-by: Kenji Kaneshige <kaneshige.kenji@jp.fujitsu.com>
Acked-by: Shaohua Li <shaohua.li@intel.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>