ACPI: update comment

Fixing wrong description for acpi_gpe_sleep_prepare().

acpi_gpe_sleep_prepare() had only used on power off and was changed
to also used on entering some sleep state. However its description
isn't changed yet.

Signed-off-by: Satoru Takeuchi <takeuchi_satoru@jp.fujitsu.com>
Signed-off-by: Len Brown <len.brown@intel.com>