ACPI: fan: Fold two simple functions into their only caller

Both acpi_fan_has_fst() and acpi_fan_is_acpi4() are called from one
place only, so fold them both into there caller which yields slightly
leaner code that is somewhat easier to follow.

No intentional functional impact.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Link: https://patch.msgid.link/3386797.aeNJFYEL58@rafael.j.wysocki