ACPI power management fix for 3.18-rc6

This is just a one-liner fixing a regression introduced in 3.13 that
broke system suspend on some Chromebooks.

On those machines there are ACPI device objects for some I2C devices
that can wake up the system from sleep states, but that is done via
a platform-specific mechanism and the ACPI objects don't contain any
wakeup-related information.  When we started to use ACPI power
management with those devices (which happened during the 3.13 cycle),
their configuration confused the ACPI PM layer that returned error
codes from suspend callbacks for them causing system suspend to fail.

However, the ACPI PM layer can safely ignore the wakeup setting from
a device driver if the ACPI object corresponding to the device in
question doesn't contain wakeup information in which case the driver
itself is responsible for setting up the device for system wakeup.

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