ACPI: Print message before calling _PTS

Make acpi_sleep_prepare() static and cause it to print a message
specifying the ACPI system sleep state to be entered (helpful for
debugging the suspend/hibernation code).

Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
Acked-by: Pavel Machek <pavel@suse.cz>
Signed-off-by: Len Brown <len.brown@intel.com>