ACPI: ibm-acpi: prepare to cleanup fan_read and fan_write

This patch lays some groundwork for a fan_read and fan_write cleanup in the
next patches.  To do so, it provides a new fan_init initializer, and also some
constants (through enums).

Signed-off-by: Henrique de Moraes Holschuh <hmh@hmh.eng.br>