ACPI / bus: Correct the comments about acpi_subsystem_init()

In acpi_subsystem_init(), function acpi_enable_subsystem() is
called to do the real job. However with different flags passed to
acpi_enable_subsystem(), different code is executed.

In acpi_subsystem_init(), with "~ACPI_NO_ACPI_ENABLE" passed in, it
will only switch over the platform to the ACPI mode. The remaining
part of acpi_enable_subsystem() is done when acpi_bus_init() is
called.

So the comments above acpi_subsystem_init() is not exact, change it
here.

Signed-off-by: Baoquan He <bhe@redhat.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>