acpi: Fix trying to enter NONE state

In acpi_system_eventhandler_sleep(), I forgot to update
ACPI_STATE_UNKNOWN when switching to new sleep types.

Reviewed by: olce
Approved by: olce
Fixes: 97d152698f48 ("acpi: Use sleep types defined in sys/power.h")
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D55119