ACPI: ibm-acpi: fix and extend fan control functions

This patch extend fan control functions, implementing enable/disable for
all write access modes, implementing level control for all level-capable
write access modes.

The patch also updates the documentation, explaining levels auto and
disengaged.

ABI changes:
	1. Support level 0 as an equivalent to disable
	2. Add support for level auto and level disengaged when doing
	   EC 0x2f fan control
	3. Support enable/disable for all level-based write access modes
	4. Add support for level command on FANS thinkpads, as per
	   thinkwiki reports

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