ACPI: clean up acpi_enter_sleep_state_prep

Remove some redundant code from acpi_enter_sleep_state_prep() and clean up
a comment in there.

Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
Acked-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Len Brown <len.brown@intel.com>