ACPI: correct id for fixed buttons

ACPI_BUTTON_HID_POWERF was changed, but this change was not propogated to
button.c, thus breaking detection of fixed power and sleep buttons.

Signed-off-by: Alexey Starikovskiy <alexey.y.starikovskiy@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Len Brown <len.brown@intel.com>