ACPI: fix OSC regression that caused aer and pciehp not to load

Executing _OSC returns a buffer, which has an acpi object in it.
Don't directly returns the buffer, instead, we return the acpi object's
buffer. This fixes a regression since caller of acpi_run_osc expects
an acpi object's buffer returned.

Tested-by: Yinghai Lu <yinghai@kernel.org>
Signed-off-by: Shaohua Li <shaohua.li@intel.com>
Signed-off-by: Len Brown <len.brown@intel.com>